I'm just going to write up a few tips/tricks that i use in the hopes that; they will open up a discussion on the best way to approach different facets of GE:S, give other players tips on how to play the game, to hear any criticism you may have or if you know of a better way to play.
For now i'll simply use point form to relay a number of different ideas which can be used conjunctively.

Opening Doors: when you open a door, don't just stand in front of it trying to get in. This give players on the other side a chance to hit you a few times before you can fire back. Open a door and strafe to the side. Why is this important? because if there is a player on the other side they will have a much smaller firing arc whereas you will have to survery the entire room to find out where an opponent might be.

Weapons: Shotgun. Use this gun most often on enclosed maps where you can get up close to each other. The shotgun does so little damage from range due to spread that your better off not fighting, running away to find a better gun (if you don't have one, if you do take that out instead) or luring an opponent closer to you.
When the enemy is out of effective range of shotgun, chase after them for a bit, or take any shortcuts you know of to close the gap. Theres really not much sense shooting when it won't do anything. (in this situation anyway)

If the enemy is using a shotgun, be careful chasing after them unless
you have one also.
When you approach corners with a shotgun and melee weapons, get as close to them as you can so when you round them anyone you see will be closer and your shotgun more deadly.

Rifles: KF7, AR33, Phantom etc.. Try to use these at range. The zoom function on some of them gives you an advantage.
When rounding a corner, move as far away as possible from it. that will make ambushes less effective and reduce the amount of damage you take from proxy mine that could be there.

For now that's all i can remember/feel like typing. If anyone reads this and finds it helpful i may bother to write more

Furbs

I also choose Automatic Shotgun over Shotgun. Always. The spread isn't so bad.

Some more thoughts.

How to move: Don't always move in a straight line, if your being shot at, duck and weave to make it more difficult for other players to hit you. Try to avoid patterns in your movement also to make it even more difficult.

Cover: Use cover as often as possible when fighting. always try to get behind something to reload then pop back out to attack. Use unusual timings with your reloading, don't wait till the mag is empty, shoot a few times, reload and continue firing. Use the time your opponent is reloading to close the distance if you think it will help you win or to get a better angle on them.

Running Away if you know your going to lose and dont' want to die, run away. If your in an open area like many parts in temple strafe back and forth to make it more difficult to be hit, running in a straight line is a surefire way to get killed. With smaller more enclosed levels/areas, when you know your opponent is a decent distance behind you it's fine to run in a straight line, just try and take the shortest possible distance to the next corner and safety fora few more seconds.
When you find body armour, or a better gun and feel you can take on your tail turn around and attack. Try to ambush, or come out from behind a corner, shoot a few times, duck back behind cover while your opponent shoots. Open or close the gap depending on what weapons each of you are using.

Aiming: Aiming always helps but you don't need to constantly aim in a fight, nor should you be trying to because it slows you down and makes you an easier target. What i do is aim for a second to get a better idea of where my reticule is then continue firing and dodging. If you have a better chance of winning, have or are about to pick up body armour you may wish to aim the entire fight because that is probably an easier thing to do.
You don't always need to aim though, when up close it's not really necessary unless your using the golden gun with no ammo

Again, these are just random thoughts/things i occasionally do. Every situation is different and you may want to go with whatever feels most natural.

Not sure if you guys know about this, but wall strafing gives you a speed boost...have to move/run away in a straight line by following along the walls, but its definitely a benefit worth explioting when you're trying to escape from danger or trying to catch up to someone you want to kill "freight train style".

I never knew that, that's a good idea.
Does your aim improve if you're aiming? Like, are your weapons more accurate if you've got the reticule up?
My tip is to bind the aim button to your right mouse key, so you can use your natural RMB instinct to aim like you do in a lot of other FPSs (COD, MoH, etcetc).

Yeah. I use the right mouse button for aim too.

I have a little button next to my left mouse button, so I make that as the Secondary Fire button. Great for those remote mines!

Im sure we've been over this stuff before.

Did you read the whole thing? There is some valuable stuff in there ^^ not just my comment.