It's pretty simple.
Since the time that CounterStrike EXP was lowered on garena.com, the only good alternatives are:

a) playing COD4
b) Playing a WARCRAFT (the Frozen throne) custom map called Fufu United Ninja AllStars
all you need is to host the game with 12 players and click 50 kills
Games finish around 6-7 minutes, repeat process for as much as you want

See your EXP boost like crazy
Tried and tested
Proven by a lot of level 50 accounts

Download the map at www.getfufu.co.nr or www.fufuportal.blogspot.com

More mirrors at:
http://epicwar.com/maps/66879/

If you have more tips and tricks, feel free to comment.