the best way to rank up is to play team death match and revive often, this will give you extra XP even with out you killing any one. Just rember to make sure there are no guys around to shoot you while you rez someone.
here are some extra tip's:
.make sure to learn where all the most used sniper spot's are
.always try to keep a good amount of ammo
.if you are going to revive your team mate make sure there are no enimies around.
it's best to stick to one or two classes so you can perfect them, using to many classes may throw you off and cause your team to lose
.try to make sure you win at all time's it may sound obvious but you get extra EXP for win's.