To level up faster than usual, try to use the same gun until all the 
marksman and elite challenges are completed. They provide huge boosts and 
will quickly level you up to get new guns. However, do not use those guns 
yet; just keep on the challenge until it has completed. The third phase of 
the challenge usually ends with 1,000 experience points, which usually 
increases you a rank in itself. Also, do not forget about the strange 
challenges like jumping 15 feet and surviving or falling 30 feet to your 
death. To complete the fall 30 feet to your death challenge if not 
previously unlocked, play the "Countdown" map, and jump into one of the open 
missile silos. You will get instant experience points for simply dying. 
Also, remember to go prone and crouch to get those challenges as well.