It depends on the kit.
If you want to level Assault/Support quickly, join a Damavand/Metro map; find the point where each team is camping chokepoints, and spam health or ammo pickups. As Assault, if you have the underslung grenade launcher, just spam it always; if Support sometimes just lay down fire (full 100-200+ clips) into a chokepoint for free suppression points.
Engineer is harder, unless you have a friend. Repairing a tank constantly with a friend (or someone that can hear you) can net alot of points. Repair is key... I've had 9 repair patches in one map, in addition to kills from the MG/etc.
Recon; you are pretty much screwed from being any type of support (ie doing something non-sniper for points). Using the T-Ugs gives no points, the Soflam is worthless except in rare cases of team participation/unlocks, and the MAV from constant/intelligent use amounts to 4 kills worth of points if used an entire map. Kills are the bread and butter of levelling Recon.