What works against snipers for me most of the time are two things (if not first killed by sniper, that is

):
- Stay low.
- Don't move too slow.
- If you've located a sniper, flank him (and if it's urban, go from building to building to get him).
- Always try to get up close and personal with a sniper if you can, most of them hate that.
- Don't cross an open terrain of not necessary (duh!).
- Try to move close to any cover you have till you've reached your point (when in a street you've eliminated one point of altitude to get shot from).
- If you can't take a clear shot at him, keep on the move till you're near him.