if shooting is an option (completely enclosed space only of course), then make the most of what space you can. for the smallest space just run at a steady pace from one side to the other, shooting single shots at a single target. using single shots will conserve paint and put more emphasis on aiming correctly, rather than starting a stream and adjusting once you see where it goes.
if you've got more space, start in the middle and practise running to the left and the right, and mix it up by having a buddy call which side to run to. you can add another challenge by setting up 3 targets and when running side to side and have a buddy call which target to shoot each time.
If shooting is not an option its pretty difficult, but can give you a quick suggestion i've seen people mention several times over the years which could give limited success. tape a laser pen on to the end of your barrel and practise running and keeping the pointer on a fixed spot as you move. this won't help with your actual shooting accuracy because paintballs behave differently in flight, and require you to compensate where you aim as you run so be ready to practise this later at the field. However the idea is if you can keep the pointer in relatively the same place it will use the same muscle groups to keep your upper body still while you do the heavy work with your legs so practising while shooting should be easier.
and if running isn't an option at all then just hold your gun while doing some aerobic type stepping, using stairs, a chair, box etc. and focus on keeping your upper body as still as possible, again strengthening the same muscles you'll need for the real thing. there's a video by thomas taylor of exactly this on youtube if you search for it.