Just some basic tips, I'm not that good myself so my word isn't gospel.
-Play with and against people better then you if possible (searching MLG or Arena should do this).
-Watch MLG Pro's streams.
-Go over your own gameplay in theater. Look at what you could have done to better help your teammates, avoid dying in certain situations et cetera.
-Ignore k/d, playing selfishly to inflate your own stats is often detrimental to the team effort so the sooner you focus on w/l over k/d the better (this does not mean mindlessly rush the objective).
-Keep your composure, nerd raging never achieves anything other then making you play stupidly which leads to unnecessary deaths.
-Then there are the basics, improve aiming (important but aiming will only get you so far. Playing smart is by far the most important aspect of a good player), learn weapon timers and the such.