I forgot to add something about finding cover: Often the best place to hide, is behind enemy lines, preferably at the very back of the map. When you have your back to a level boundary, it's just one less side you have to worry about. In big maps that are often sniper vs. sniper duels, swap sides; meaning you go on your enemies typical side and hiding spots and snipe down their line.
Hehe every match I play I come up with 3rd, 2nd or most kills =D