Werbal gives some good advice. Here's some more, for my personal best class, the Sniper.
- Your laser sight is a dead giveaway. Always, ALWAYS, keep it trained on something close to you until you actually go for a shot. Remember, your enemy almost always can't see you, but they can see your laser dot, and they'll avoid it like it was herpes. Find a windowsill, barrel or anything else in your close proximity, and hunker down with that in the field of view where you're actually aiming. On a map like Dustbowl, keep it trained on the wall to the right or left of the tunnel you're covering.
- You only need one good Sniper. I know everyone wants to be a badass, but you're a defensive class, first and foremost. More than one good Sniper is taking away a valuable body count from your offensive coordination.
- You don't need to find high ground to be a good sniper. Frequently, you'll get the most shots on the ground, using a building for cover. This is especially true on the various Dustbowl maps.
- You are expendable. You're going to die a lot. Deal with it. If a Medic is healing you, politely tell him to go be useful or die in a fucking fire. If a Medic ever ubers you, curse his lineage until the end of time. I've had to yell at far too many Medics who'll just sit with a full uber meter on 2Fort just to make sure I stay alive. It's dumb. Don't let it happen.
- Your natural enemy is the Spy. You can't do anything about it. Sorry. If a Spy gets up to the Battlements on 2Fort, you're fucked.
- Scouts are extremely difficult targets, except on 2Fort. Learn when and where they'll make that jump to the top of the bridge, and you can pick them off in mid-air. Even if it isn't a headshot, one full charge shot from the rifle is enough to kill a scout, and almost any other class aside from Soldiers and Heavies.
- Destruction. You may not think you're a good class on Sentry detail, and you're not the best, but in a pinch, your long range, and the power of your shots, can be invaluable in taking out sentry guns. If you see an enemy Engineer setting one up, shoot it right away. Then shoot him. Then shoot his friends. Then attempt to shoot your friends to make you feel better.
- If, at any point, you have to resort to using the submachine gun, you have failed as a person and you are already dead.
- Most importantly, patience is key. Find a good spot, stay out of sight, and STAY ON DEFENSE. I know that on the 360, the maximum game size is 16. A sniper on an offensive team when you've only got eight slots is crippling yourselves. Find another class to be good at.