I find that just pressing the belt until it touches the upper deck, then rotating the drivetrain does a pretty good job. It only takes a couple minutes of doing that to loosen it up a lot. I've seen guys put the belt tensioner on the car and crank it all the way down, then run a few packs through. I think that would put a lot of extra strain on the motor, so I wouldn't recommend that method.
Practice is good too. Just work on being nice and smooth out of the turns, getting a good launch onto the next section of track, and taking a tight line on the infield.