Big thing most people forget about being a sniper is that you have to be aware of your surroundings while you're aiding your team. Lots of noobs will find a place to camp and then just stare down the black tunnel of the scope and think that they're invisible because they're at long range.
This is just not true. People will get agitated at being picked off and they will find a way to flank you if you don't do a few things. (Sorry if these are repeats)
A) Find your favored range. Lots of people will think that long distance sniping is the same as medium distance. Not true. Got to do different math for different distances, plus there are better weapons for each distance. Once you've found your favorite range, base the rest of your thinking on that.
B) Find several good spots. A good sniper camps, yes, but he does not camp in the same place for more than one or two (or in good cases, three) kills. As the guys above have said, they'll find you. You want variation. Keep them guessing and they won't find you.
C) Know your area. Do not ever just stare down your scope. Keep an eye out on the area around you for the odd Kenyan or Ghost who will flank the team and try to surprise people with a knife in the sensitive parts. Break from looking down the sights to check every so often. Also, if you can, try to match your ghillie with the brush around the area just in case. That's bought me a few extra seconds before.
D) Do not rush the enemy with your loadout. You are a sniper. You are not CQC and you never will be. Thinking that you are, especially with quick-scoping gone, will get you laid out faster than you can think "Bonzai!" This should be obvious, but it is something noobs do every day.
E) To lead or not to lead? As I said before, there's different math for different distances, so this may not be exact, but usually it's a good idea to lead your target slightly. Modern Warfare/COD tends to have better physics than Halo, but Halo has sprinters, so it's all about the same. Experiment and find out.
F) Take out enemy snipers first. Make these guys your priority, or else they'll get you. If you can eliminate their snipers first, you can easily pick off the assault/engineer/stealth guys milling around in the field. Check spots that you know a sniper might be first before taking out the little guys.
G) Have fun. I know I probably made this sound super-srs, but really it isn't. Don't ever forget it's a game, and to that end, you're supposed to have fun. If you die, oh well, you learned something about what not to do as a sniper.
That's about it for my rant. Hope it is helpful to you and others. Sorry if I repeated what other guys said, but there really are only a few tips out there, so it's bound to happen.
Happy trails.
