I'm pretty average at shooting and skulking when actually good players are around
, but all I can tell you is becoming better just comes with experience.
You don't even have to shoot that well to be decent. You just need to spam "hot spots" and learn the ways skulks move on marine side. There's only so many things that a skulk can do to surprise you. They can wall-jump, leap, jump, and strafe. While a good player makes those reasonably unpredictable, you can still learn a few trajectories from experience, and when you get the timing right you can even start prefiring around corners based on predictions from what was seen on the map.
It helps to also learn how to switch to your pistol after depleting your AR mag. The pistol does reduced damage with armor, but huge damage without armor, and usually a skulk's armor will be down after you've finished your AR mag (unless you failed hard lol), so usually all you need to do is land 1-2 pistol shots to finish him off. Switching to pistol is much faster than reloading the AR, and is easily the difference between life and death in the split seconds of a fight.
Of course, learning to time jumps is extremely beneficial too, in the unfortunate situation when the skulk is next to you - and it helps your teammates see/shoot him better.
Generally you should also try to maximize the distance between you and the skulk. Unfortunately when marines are in a losing position, you HAVE to push, and that's when it gets ugly because you can then be ambushed
. Of course, try to always check the ceilings and corners when you go through a door.
1v1 is sometimes unavoidable, but it's not the ideal situation. You should usually if not always have at least 2v2; then you can do some things like learn how to put space between you and the other person, so you're not far enough that you can't see each other, but also so when you get attacked, you're not both huddled together. If you get it right, usually it's a matter of spamming around the other person's feet as he's jumping around :-D .
When you get jetpacks, it should be much easier, because you can kite everything - the backward movement is no longer restricted, so when a skulk is running at you just fly backwards as you shoot him. Same with fades, lerks, oni, etc. You just have to know the maps so you don't run into a wall.
Um, that's about everything I can think of right now I guess.
P.S. Oh yeah, as in any game, getting at least 45 FPS helps a lot. It's pretty hard to accomplish that in NS2; that's why people with better PCs generally do better...