Changed my button layout to Bumper Jumper back during halo 3. Allows you to jump and move while never taking your fingers off the stick. Can make some abilities like jetpack hard to use, so I just don't use them. Bumper Jumper is excellent for swat to keep a moving target and still be able to hit people while moving.
Crouch. Always crouch to stay off the radar, helps to loose people and to get the jump on people.
Avoid heavy vehicles. Hijacking vehicles is fun but can cost you a few deaths to finally take a mantis or tank down.
If your not good at driving certain vehicles stay out of them, let someone else that lives for the banshee take it if all your gonna do is a loopty loop to your death.
And finally stop dying. Pick your battles and stay out of the middle of battle. Help teammates by assisting or stealing kills. I am happy with a game win or loose if my kills are more then my deaths.
Check out my Halo 4 vids via search for Minstogo in halo 4.
I would like to add to this. Play with friends. When I play with friends I am usually the one leading assaults for I better aim than most of my friends and they pick off shots from the back. If you are playing with randoms, what I like to do is stick by one person at all times, doing this when playing with randoms has allowed me to pick people off from a short distance behind my team mate and has helped me, as well as my teams, quite a bit