Well it depends if you want get better as just a player or the whole team get better.
If you personally want to get better then practice your snap shooting. One of the easiest ways to do this is line up some old coke cans, then get behind your bunker and just pop out to shoot the cans, but dont stay out to long. To make it more interesting you could get one of your buddies to shoot at you as well, just so you dont stay out that bit to long, but all you are doing is shooting at the cans not your buddy. You will really improve after a while, but be prepared to get your head shot off for a while until you get better.
And as Quayler said being fit helps as well especially towards the end of the day.
Darm