I would say avoid big games especially Ground War. It seems it's easier to get kills because there's more people but in fact it's a lot harder to get high kill streak because of bigger chance of getting killed.
Do learn the maps as fast as you can so you can easily pick good spots and avoid those from where you can get killed from easily. Watch killcams for that (as was suggested in the answer to another question).
Unlock challenges for the same gun to get optimal performance out of it. Almost every gun (at least from assault rifles) can be configured to be effective.
Lastly, try to play stealth to avoid many deaths and therefore build higher streaks. Use silencer, Ninja and Cold Blooded for that.