On sniper matches, if someone spots you when you're in cover, put your head in front of one of the gaps and get them to waste shots. For some reason, they seem to think there are sensors in your eyes and teeth (
just because they glow doesn't mean you can shoot them! ) Also, there are now 4 and 1 minute warnings where you can't be shot. If you're willing to get shot once, you can take shedloads of shots without any of them registering. I drained a certain someone of a ton of shots doing this. Of course, both of these only work if you're borderline insane, like me, and will more than likely get you shot a few times anyway.
When you're out of shots, your gun will make an annoying noise instead of a normal shot noise when you try and shoot. This is a giveaway to anyone else (
and also provokes me to shout FUNNY NOISE! but that's not the point) and you're likely to have everyone and their grandma chasing you for a while, so make sure you keep an eye on your ammo. It'll show up when you have less than 50 shots left, I think.
Try and shoot one handed at close range. This gives you one hand to balance, and I'm personally more accurate when I'm shooting with one hand anyway, even at long range. I know Wolfie knows I take Sniper too literally in that sense.
And one REALLY important tip: Don't wear white. I don't think I need to explain this one.