Marines need an infrastructure just as much as you do, so taking time to run around detroying unprotected stuff (especially res nodes) is usually quite rewarding. If anything, it causes a few marines to have to sidetrack to take care of you. If they don't, you just destroyed their stuff. Congrats.
Forcing the Comm to beacon is also good. It means the Marines are now back at Marine Start instead of somewhere else, killing your dudes. Usually it's customary to go for the phase gate so they can't get BACK to where they were, killing your dudes, but ninjaing the Arms Lab, Adv Armory, or even the Observatory help dramatically since usually the entire team is relying on only one of these structures. Then they have to take the time to rebuild one, giving your team breathing room to deal some damage. However, this does not mean sacrificing yourself for the "greater good" especially as an onos. That's 75 res for moving the marines around a little and maybe causing 30 res damage. So, if you hear a Beacon going off (it should be obvious), you probably want to start running.
Skulking Tips Keep Moving - When in battle you should not stop moving/jumping while in range of marines. It takes marines roughly half a second to aim and shoot you, if you aren't there, you may live. Stopping to make sure you bite your target will be less successful then biting marines at random.
Zigzag - Almost as important as keeping moving: if you're not going to leap in, a skulk simply does not have enough life to run straight in if the marine is competent enough to anticipate your movements. In general, zigzagging is a fairly effective simplistic pattern, but be careful not to be too predictable: for example, whenever I knew I was fighting MortosDerSoulstealer, he tended to run a fairly predicatable pattern where he would run at you, and then dodge by heavily strafing in one of two directions: it was very effective until I caught onto it, which let me pretty much predict where he was going. Try to avoid patterns like this. (Obviously, this is no offense intended to Adam, who is an amazing player, but it was the best example I remember of this.)
Groups - When encountering a group aware of your presense, jump over the ones in front and attack those in the back. Usually they will not expect this to happen, and may be unprepared.
Rocket Powered Skulk - When xenociding, use scent of fear to identify groups of marines, then active xenocide, immediately turning the corner and leaping at them. This is a wonderful way to catch them unawares.
Gorging Tips And this cannot be stated strongly enough: early in the game, use lame to secure crucial junctions: it can cost marines valuable time and allow the aliens to secure the vital early advantage they need. However, building lame at the expense of getting Res nodes up is tenuous at best, and usually only useful if you have support to hold that uber critical juncture.
Getting up chambers quickly is of paramount importance. Sensory has its peak effectiveness early game, and drops dramatically quickly. The other popular early choice is movement, but ask your teammates what they would prefer (movement is FAR superior if you can get an early Fade). Build 3 of a kind of chamber to get maximum upgrades of that kind. Make sure to build a few extras in case marines take one out!
How to go sensory first Sensory can provide a critical early-game advantage if used properly: put up a network of sensory chambers covering large portions of the map. Build 1 in each hive (gullible marines may walk through the hive location without realizing it's there), and then others just far enough apart so that their ranges barely overlap. As a gorge, get Focus. As long as you stay within your SC network, you can see marines coming on scent of fear, and are cloaked. Spit them once and you've dropped them 60 HP from long range without them knowing you were there before. 2 focused spits kills a level 1 marine. Make sure to take advantage of cloaked lame, as well: marines won't see it until it has already shot them once. If you have a good higher life form on your team who wants it, go movement second; otherwise, go sensory second. SCs should be dropped in corners, large vents, or other places where marines won't walk into them or randomly shoot them.
How to go movement first This is really only useful if you have good offensive players on your team, and it puts the game into the hands of the fighters, not the builders. It will make early Fades amazing; early Lerks will also be better, and it can improve skulks as well. Just build MCs in hives; they are not that useful elsewhere. Then you should probably go follow the fade and heal it. Go defense second. MCs can be built anywhere; being hidden is helpful, but marines usually take them down last.
How to go defense first Defense first has gone out of style recently, but it's a very solid and reliable beginning. Build 3 DCs in each hive location in the most sheltered places possible. This will make the hives very defensible. Build an OC on either side of a doorway (on the inside) with one or two DCs behind them: this will make it very difficult for marines to Rambo. Build healing stations of 1 DC a little ways back from key locations so that higher lifeforms can fall back and heal quickly (since 3.2 only 1 DC affects the alien in range, but heals the equivalent of 3 DCs from pre-3.2). If you don't need to build elsewhere, stay at the healing station to provide further backup for higher lifeforms. If you have a lot of battlegorges and are generally playing a building-heavy game, go sensory second. Otherwise, go movement second. DCs should be built low down, in vents, and other places where they're hard to shoot but aliens can get close to them. When possible, put an OC above and in front of the DCs to draw fire.
How to build OCs (AKA lame) Early game, build OCs only when you have excess res. Drop 1 in front of an alien res node to defend it. Drop others at doorways to hives and key locations to prevent rambos. OCs have limited range, so don't build them at the ends of long hallways. Instead, build them places where marines will be surprised by them. Build pairs of OCs on opposite sides of doorways, etc, so they can both shoot at a marine at once, but he can only shoot/nade 1 at a time. Don't build single OCs places where they'll be easy to take out. Late game, build walls of lame. Marines will be highly upgraded enough to take out single OCs, so build them in groups with DCs behind. Try to keep them far enough apart so marines can't shoot/nade them all at once. Use lame to block heavies and back up higher lifeforms. Lame should be built high up in places where it has good LOS, but where everything is still in range.
It should also be noted that there is one other situation where it is wise to build early OCs: I've found that a piece of lame or two is an excellent way to hold down the doubles early game, especially in Eclipse: If you climb the wall as a zergling into the area overlooking the rest of reactor room, a couple of pieces of lame up there can be very annoying and impossible to remove early game. Well-built OCs in double can be worth more than the res node early, as marines will often rush these locations as well.
It should be noted that the competitive league players tend to go MC -> SC so that they get celerity focus Fades. They're so l33t they don't need carapace when they've got metabolism. Unfortunately, Crack in the AC players aren't all l33t, and they tend to wank if they don't get their DC for second hive.
Don't expect one early gorge to be able to do everything you need early game. Don't be selfish and expect everyone else to Gorge.
There are two ways to Lerk.
1) Support: this is usually the best use of a lerk. Since you're essentially a flying skulk with worse dps, you shine by the ability to gas stuff. This means throwing spores around and injuring marines before they get into combat, making the lives of every alien much happier. Once you get 2nd hive, Umbra = OMG hacked hitboxes. Actually, it just reduces the number of bullets that hit by 50%, making Oni and Fades almost invincible when done right. Also, it lets you fly into exposed areas, shoot out that gas, and run away without getting slaughtered by the 3-5 marines you're gassing. Take adrenaline, though, because gassing takes alot of energy, and scent of fear lets you see where to aim before you turn that corner. Also, try to put the gas cloud in front of the marines so they have to walk all the way through it, maximizing your gassing damage. For umbra, put it where you or your friends will need it most, such as on the marines you're about to attack.
2) Hit and Run: For the advanced lerk player, you suddenly realize that since you can fly, you essentially are like a 2nd level skulk with leap. Of course, you also cost 30 res more. However, this means that by mastering lerk flight, you can essentially harrass marines and dodge most of their fire. Note that this requires near mastery of lerk flight. However, by taking either adrenaline or celerity, and perhaps focus, battle lerks are a force to be reckoned with especially in early game.
Lerk Flight: Lerks can fly. In order to gain forward propulsion and upward motion, tap the Spacebar repeatedly. However, once you reach your optimal speed, simply hold the Spacebar and you will glide. Trying to go up will reduce your speed, as will turning, at which point you tap the Spacebar again. Being able to stay at top speed with minimal adrenaline consumption means getting hit less and letting you do more before running away to recharge.
Fading Tips Hit and run: No matter what you may think, the label "higher life form" does not make you a tank. You're effectively a skulk with enough life to survive most encounters. You can tank the fire of one (or two with bad weapons) marine, but your life drops a lot faster than you may think. Learn to love blink: it's the heart and soul of a fade. Blink into a marine (or group), get a couple of slashes in and kill one, blink right back out (if necessary, but usually will be), use metabolize to regenerate life, repeat.
Ducking: the common strategy when fighting fades is to crouch as a marine, ducking you out of the hit box of the attack. Look out for this, as it's very common, and can allow a marine to solo the fade if he's not prepared to deal with it. Often you will have to aim lower than you normally would.
NOTE: Although it seems like for your swipes there's a delay in the attack, there is in fact NO DELAY. When you click the mouse to fire, that's when the damage is dealt, not when the animation takes time to wind up and then swipe. So, don't try to anticipate and click early. Get into the marine's face, click to swipe, and get out before the animation finishes.
Onos Tips You are the meatshield. With 700 starting max health and 600 armor, you can soak up the most damage of any unit in the game. However, this comes at the price of a giant hitbox and speed that rivals only the gorge. However, you get one really cool ability: the ability to devour marines whole and digest them for health and adrenaline. Yummy. Plus, it doesn't matter what their health is, or if their in Heavy Armor. They're just.... devoured.
Another important fact is that they WILL shoot you first. Mainly because you can do serious damage, but also because you're the easiest to hit. You'd shoot the giant elephant charging your way before the little birdy. However, this can work to your team's advantage by you drawing fire while the skulks and lerks do the dirty work by running between the marines' legs.
Also, you take 50% damage from turrets. This makes you the best choice to take down static defenses. That and your giant amount of health.
One last thing, you also get stomp. This stuns all marines in a straight line for a while unless they have jetpacks. This means no shooting, and no moving. It also means it's easier to devour them. It also takes up a bit of adrenaline. Use it wisely.
Take celerity. You need the speed. Unless you're going to be in an extended engagement or have 3 Hives, in which case Adrenaline means more hurting and/or Charge over a longer period of time. Scent of Fear is probably a good choice too, although getting devoured by a cloaked onos is priceless. However, DCs make you happy. Very happy. Carapace means more tanking, and redemption almost always goes off due to the enormous range that it triggers in (the <40% range = below 400 health? that's a lot of chances). Regneration is good against light amounts of damage so you can steady-state it or just shake it off on your way to your next meal, but not very useful against heavy firepower.