Go through the weapons. If you prefer assault rifles, go though each one and master it. Same for submachine guns, MGs, pistols, etc. When I say "master it", I mean that you unlock all the attachments and get 500 kills. No need to waste the time getting the 10k points for the 1000 kills. By that time, you could have got another 500 kills with a different assault rifle and gained 30k plus points in challenges alone. A lot of people don't go for the Extended Mags attachment, but it's easier than you think. There is always a sniper or camper hiding behind penetrable cover. By the time you get 300 kills, you will unlock the FMJ. Equip the FMG and go for the 500 kills. By the time you get 500 kills, you will have gotten the 40 FMJ kills. That's at least 20k points right there (10k for master, 10k for 500 kills). As you rank up, just look for the those challenges that you've been unknowingly progressing on. Finish em off for quick points.
That's what the high level players are doing. Eezy peezy, eh?
Oh, and winning definately helps you out ^_^
Add me if you need a ride to victory GT: gorillaCHEEZ