Humping the armory, dont do it – This is a phrase describing marines that will hang out by the armory and fill up on ammo before heading out somewhere. This wastes valuable time that could be spent killing things. In general, using the armory twice as you pass it should get you all the ammo you will need before dying. If a fight is going on nearby, or through a phase gate, get no ammo. Every second is precious, and most likely there will be new ammo or guns lying on the ground there anyways.
How to kill offense chambers – These chambers launch fast moving spikes at marines from the ball on their top. If you come across them and have no grenades left, there is a trick to killing them without dying yourself. If they are far enough away that you can get out of their range, then just stay at range and shoot; you may need more ammo, but that is what the comm is for. If the chamber is in any way behind an obstacle, or around a corner, then position yourself so that you can just see the edge of the chamber, but not the top that shoots spikes. From this position you can shoot it without taking damage in return, and slowly remove chambers from the map.
If there is a gorge healing the chamber, you probably don't have enough ammo to take them out unless they are already damaged. If you need it dead, or don't mind dying yourself, ignore the towers and suicide charge the gorge. Jump over or run around the chambers as fast as you can and unload you ammo into the gorge. If you kill it, then the next marines that way can clear the chambers easier.
How to snipe – The pistol is a very strong weapon when used correctly, especially against less evolved kharaa. It can shoot as fast as you can click the mouse button, allowing 10 bullets to be shot in very short amounts of time. Sniping refers to using the pistol to kill an alien at range before they can react. First, practice shooting the pistol to the point where you can click the button quickly without moving the crosshair. Second, be patient and aim. If you know a skulk is going to be coming around the corner, or he is parasiting people from a distance, draw a bead on him. He likely will wait until you fire a shot before running, expecting lmg spray. Go ahead, let his parasite hit you while you aim; then fire off all 10 rounds at his location, and your skulk/lerk will be left wondering how they died so fast.
Recognize distractions - Gorges, lerks, and skulks in vents may, instead of doing actual damage, prefer to snipe a bit, then hide in the vent. If clever sniping or grenades doesn't work getting them out of there, just continue on. Three marines wasting time to get a single lerk out of the vents is a net 2 player advantage for the aliens, don't fall for this.
How to block – It is common for lerks, fades, and oni to use hit and run tactics when they can't handle the force of the marines all at once. If your teammates aren't stupid and actually shoot the lifeform as it passes, but can't seem to get enough damage to kill it, blocking becomes the best strategy. The aliens stop instantly when they hit a marine, so before their next charge through, position yourself by the exit, and before they fly out, jump in their way. Continue shooting, duck if they try and kill you, and don't let them get around you. You almost certainly will die, but those extra seconds should be all your teammates need to finish off the beast.
How to use motion tracking – The most important use of motion tracking, and will make you look very badass, is noticing not where the circles are, but remembering where they where when they vanished. Any circle that vanishes nearby means either an alien has stopped moving there, or has cloaked. These guys now think you don't know where they are, and this makes them sitting ducks. Try and figure out where you would be, in the terrain of the room that the dot disappeared in. Then walk in and shoot that spot. If it wasn't in a good ambush location, assume they are cloaked and walking towards you, so drop a grenade and be ready to shoot.
How to listen – There may be no more useful ability in NS than listening, especially if you are off alone. Learn to distinguish gurgle of a gorge, the roar of an onos, and the sounds of the special abilities each use. Also, learn to pick out the tic tic tic sounds of an approaching skulk over the sounds of building or knifing a node.
How to build/knife a node solo – This can be really tricky, because a nearly finished job is no better than a just-started job. Once the room is secure, start building. Face so that you can see openings, or are watching aliens on motion tracking. Listen carefully for sounds of approaching skulks. As soon as you think you hear one, step back and wait, preferably out of sight. If you choose to keep working on it, the skulk will see you and attack while you aren't paying attention, making for a certain death before the structure is completed. If you are willing to hang back until you are sure the alien is either gone or dead, it might take a bit longer, but you will be successful more often.
How to Rambo – A rambo is a marine that runs off alone, gets behind alien defenses and wreaks havoc there, either killing unsuspecting aliens (thinking they are safe so far from the marines), or building phase gates at alien hives. To do this, first you need to get by their defenses without being noticed. Warning – Rambo should not be done if you are needed elsewhere, if the comm has other orders for you, or if you are needed as part of a group (especially if you have a welder, with the exception of rambo-weldering). It can, however, help the team significantly in the right situations, so be aware of them.
Step 1: Getting around static defenses – Realize that offense chambers take a second to start firing, and don't do all that much damage if you keep moving and get behind cover quickly. It may be possible, if you keep running (or jumping) past the chambers, to get by with a minimal amount of damage. (Usually aliens don't expect to meet marines near locations with lots of OCs, so take advantage of their complacency.
Step 2: Getting around aliens – Do not attack every alien that passes. They expect this, so if you hide in corners and don't shoot, they will assume the room is clear and continue on, not giving a second thought.
Step 3: Stealth – This all hinges on getting as far as you can without the alien team as a whole realizing what you are up to. Don't attack buildings (you'll do more damage if you wait and attack buildings further back), follow tunnels and vents when you can, and try and stay off paths directly between hives and marine bases. Use motion tracking to it's full potential here, as most aliens won't bother being stealthy. Things to do once you are there
Phase gate – Get to a place out of the way, but near a hive, and build a phase gate. This really only works if you have done nothing to give away your position so far. Make sure the gate is out of sight of the hive and not near any paths skulks are likely to take out of the hive, and make sure that the room is clear before attempt. Realize that building can be heard easily, so stop everything if something spawns in. Don't kill any aliens unless they see you, or you'll give your position away to early. Watch the kills that show up, because every time an alien dies, there is a chance he will respawn at that hive.
Havoc – Also known as egg search. In this role, try and do as much damage to the alien team as you can before they kill you. Try and keep moving, kill any eggs you find, shoot aliens in the back, and avoid oni or fades (notable exception, if they are running away from marines, or redemption into the hive, they might be weak, so finish em if you can. Use motion tracking to tell when aliens are coming, pick a camping spot, shoot 'em, then move again.
Distraction – This you can do as well, usually if aliens are attacking or defending a key outpost. Camp in the hive room, periodically shooting the hive, and sniping any aliens who come in to check on it. With any luck, the aliens may panic and withdraw their forces from the outpost. Sure they will kill you, but your team will have taken the location in their absence.
Camp – Also like havoc, but less mobile. Here, pick a camping spot along a main pathway between the aliens and marines, and snipe aliens that pass (ignoring fades and oni, of course). You may move once in a while, but if your spot is good, and you are discreet, you can consistently kill half of the alien fighting force.
Destruction – When the alien forces are far away from you or busy, you can start weakening their stuff. Knife nodes, look for and destroy chambers, even take down OC's so more marines can join you later. Ideally, by the time they come back over, they will have lost infrastructure on that part of the map.