Drills, drills, and more drills! The off-season is the best time to work on the fundamental skills of the game.
Now that teams won't be dying for time on a specific layout in the lead up to an event, getting field time is much easier so it's the best time to take advantage and build on the basics - snapshooting, laning, running and gunning, sliding etc.
I'd always recommend a good training routine outside of paintball as well. So hit the gym, get running, go swimming, do whatever you fancy but keep yourself active. It's very easy over winter to become stagnant and put the weight on. You'll thank yourself for it come the new season!