That's exactly the reason I bought my trials bike, to train for off-road racing. And yes, it does help alot.
Drills: Go as slow as possible while standing. Practice sharp turns, circles, and figure 8's. Don't look at your front fender, look where you want the bike to go. Learn to stop and remain on your pegs balancing the bike. Start, stop, start, stop (when you get good, you'll be able to do a nose wheelie to stop and then wheelie of the start). Work on throttle clutch response.