TBH, I think just the amount of time spent throughout the years playing every HALO multi-player starting with CE & starting XBOX Live with 3(I played as much online 2 as I could but I was too young to have a credit card required for original XBOX Live so I didn't have a GT).
That & the occasional playing with people that are better than me helps as well.
RE4, REmake, HALO 1 & 3, & other videos here v http://www.youtube.com/user/Jaidabecca
I played with people who were better than me, and I gradually got better just because I was sick of always being the weak link. In Halo 3 I went from a .70 KD and a BUNCH of losses, to a 1.09 KD with about a .500 average on wins. In Halo 4, my KD settles around 1.90, which isn't all that great, but my win/loss ratio is really good, and that's what I go for.
Specifically what I did was stop rushing with the AR. I learned to use the BR in Halo 3, and I learned...I don't know, how to PLAY. Haha. I feel like with Halo a big part of it isn't picking certain weapons, but being able to use them all in different situations. I just became a smarter player. My skill isn't nearly as good as some of the better players, but I'm a great sniper and yeah...
Previously erksage. Mother 3 to America!....right?
Use it. People say it lowers awareness but it actually gives you an advantage because you can focus on the screen rather than the radar. And use stealth if everyone starts using pv. Use the boltshot and dmr. Crouch a lot and only move if you have a set destination in mind or in the middle of a firefight.
Oh wait, I was supposed to talk about how I improved. I realized to stop being an idiot. If I die more than 6-8 times in a game it was an awful game. It's easy to contribute if you are simply not dying. In BTB you should only die like 2-4 times a game in a win. Don't use assault weapons, they are worthless.