A'right, here's a couple of things to think about...
NS is a game of asymetric warfare. Different strategies and tactics apply to each race.
The first thing to realise is that Aliens are predominately melee attack and marines are predominately ranged attack. Consequently, aliens prefer to engage in confined spaces, corridors and the like whereas marines gain the advantage aiming down long corridors and guarding choke points.
Knowing that is easy, but applying it is hard. As a marine, say you're guarding the top entry point to Data Core. You see a gorge pop his head around the corner. You get some shots into him and you think you can kill him with just a few more shots. You may even think "I'll get a clear shot at him as he runs down the ramp to Reactor". So you chase after him only to be chomped by a skulk as soon as you get to the corner. What happened was you moved from an area where you had the tactical advantage to an area where the aliens had a tactical advantage. Now there's nobody guarding DC and the aliens can move in.
The second point is that the alien team enjoys a mobility advantage. Skulks move faster than marines, can make use of vents to safely and quickly transit between areas of the map and later the leap attack can get a skulk back to the front line a bit quicker. When flying, the lerk can move many times faster than a marine and of course the fade has the blink/teleport ability.
The mobility advantage encourages a gurilla warfare strategy: Skulks fan out and take down power nodes and marine resource towers in the early game. In the late game it means Aliens can chose where and when to engage the marines. The aliens can rapidly change the focus of their attacks, disrupting the marine strategy.
- going rambo is arguably the single worst thing a marine can do for his team. Group up, cover each other from a distance. Twitch gamers take point, the rest of us follow in a chain. Keep line of sight on your teammate, keep your aim pointed at his feet. Stay 5-10m away so that you have an easier time aiming at a skulk dancing around the feet of your commrade. If your team lacks this kind of dicipline, do the best you can by waiting for a rambo-type to run past, then tag along.
- when a skulk is dancing around your feet, you can jump around and move eratically to buy your teammates time to kill it. Also use the secondary attack with the rifle.
- After each engagement, repair your teammate's armour (aka "weld me!"): face them and press e. The welder stands behind the weldee, while the weldee guards. Don't face each other and weld at the same time and don't put your back to enemy's approach. If you hear a marine ask for a medpack, check if he needs welding.
- use short, controlled bursts. This conserves ammo and usually kills the target quicker than holding the trigger down while your try and aim.
- Skulks need to sneak up on or ambush marines. At the very least, try and use an unpredictable aproach vector. Practice running along the walls or the roof. The game makes this easy but it is a bit disorienting. Esentially you want to close the distance on the marine giving him as little time as possible to aim at you, or making that aiming very difficult. Also, chose your battles. It's on your terms.
- Skulk bite attack obscures your vision and disorients, which can make it difficult to bite an evasive marine. Borrow the marine strategy and use short controlled bursts: only presss bite when you're sure you're facing them.
- The skulk's parasite is AWESOME. Get a parasite into every building and marine you can. It assists your teammates, injures marines (two bites and parasite kills, otherwise three bites are required) and makes marines fearful of advancing. It can only be removed with a medpack, which costs marines resources and uses up the commanders time and attention.
- Situational awareness! Check/watch the map (press c), especially when building or guarding as a marine. Less important for aliens due to hive sight, but still useful for working out where to attack next or where is safe to evolve when you respawn as a skulk.
- Situational awareness! Try and keep a feel for who is alive/dead. If three marines recently died, marines should be vary warey of running out of a secure hardpoint into a corridor whereas aliens can take a few risks for a little while.
- Situational awareness! Communicate with your teammates, but keep voice chat to a minimum.
Marines should secure key map locations: hive/cc rooms. Turrets, observatories and phase gates. Marines can then launch an attack from any of those locations, in any direction and be becon'd back as required. Get the commander to scan the way ahead before you move in to a potential ambush.
Aliens use gurilla warfare and attritional damage to distract and overwhelm the marine commander. You can bleed a turret's ammo out by dancing around in front of it and running back to get healed up. It becomes more effective if you can dance around a bit to avoid getting hit and the closer you are to a place to get healed (hive/crag/gorge).
NS is pretty different from most other fps games. You don't win by having the biggest frag count. You win by pressing your advantages, exploiting the enemy's weakness and working as a team.