As le-pig says, you need to chuck tons of paint from the back. Any decent team will share the paint bill equally as there's no way your front guys will get into their positions without you shooting them in, so it's to their benefit if you have an itchy trigger finger.
You should be the eyes & ears of the front guys as well, so you need to keep alert for what the other team is doing. Don't get "tunnel vision" & concentrate on just one of your opponents, you need to be aware of what's happening on the whole of the field.
If you play to either side, especially on arena or speedball type fields (e.g. sup'air), you should be firing across the field for the majority of the time rather than just down the line. Find out what works best for you & your team, but a rough giude would be 75-80% of your time concentrating across rather than your line.
If you play centre however, you need to be able to come out & be effective from both sides of your barricade. This is obviously better if you can fire with either hand, so practice "wrong handed" shooting whenever you get the chance. This is a useful exercise for all players but especially those who play in the centre.
Communicate, communicate, communicate. A good back player, especially centre back, should be hoarse by the end of the day. Let everyone know what's going on everywhere on the fields. Repeat every call you hear. Try to keep up with the elimination count & keep repeating it.
Back to the first point about shooting volumes of paint, take out more than you've ever shot in a game, every game. The "big boys" take out 11 pots or more if they play at the back & even at novice level here in the UK I get through at least 5 pots in a decent game often more. If you are confident that you won't run out you'll shoot more which for a back player is a GOOD THING. Oh yes, & make sure you have a big enough gas bottle to shoot all that paint - it's no good carrying 15 pots if you only have enough CO2/Air/Nitrogen for 800 shots!
On the break, rather than head down sprinting to you own barricade then starting to fire you should come out firing & attempt to hit the opposition on their break, walking slowly into your own cover. Try sweetspotting on the break. By that I mean If you get a chance to walk the fields, look for their running lanes on their break & fire a constant fast stream of paint into those lines - there's a good chance you'll get at least one or two on the break that way putting you up on body coulnt almost straight away.
Be alert for your front guys thinking of making a move, then lay down constant paint to keep the opposition in when your guys make that move.