I'm pretty much a noob to this game, but I think I've finally found how to long distance snipe without dying instantly.
Never stand still!
As long as I don't stand still, counter snipers can't get a hit on me right away (1 in 10 does). The first thing I do when i find a good position is check for counter snipers. What I think is kinda lame about this game is that it's made easy by the HUGE white glare glinting off of their scope. Oddly enough, it seems most of the time they are laying down and are an easy target. I usually get them first shot and they completely miss me because I'm pacing back and forth randomly. (when their 200-300 yards anyway, beyond that it usually takes the first shot to figure out the range) Occasionally I stop to take the shot, but if I stop for more than a second and a half, that's long enough for an M98B to get a hit from +/-250yds, so I never stay still for longer then that if I think I'm in sight of the opposing side.
It seems that I always die the fastest when I am lying down in the grass or behind a wall with tons of cover. If I'm using any long range scope the white glint will give me away instantly to opposing snipers that are already in position. And it doesn't matter HOW much cover I have because good snipers can shoot you through a pin-hole in the wall.
Oh, and another thing I've noticed is that most people will stop what their doing and go out of their way to kill a sniper if they see one up in a good position with tons of line-of-sight. I actually avoid the typical "top-of-the-building" vantage points because people check them so often and will throw all the explosives they've got up to where you are until they see your body flying. Then you either die, cower in fear until the wall separating you from their bullets is gone from rockets, or take the noble suicide jump from your comfortable sniping nest.