Treadmill is simple enough: You set up two teleporters in a straight line, say one to the north of the other. You then stand between them and slowly start walking towards one... Step on it, get sent to the other... continue walking in the same direction and the process repeats. It's a treadmill.
Remember, the more attempts you make to hide, the more chances you get to gain. Activating stealth (by walking while hidden) resets your skill wait timer down to about three seconds (as opposed to the ten seconds you'd have to wait if you were using hiding on it's own).
Regardless of whether you succeed your stealth check, attempt to re-hide as soon as the skill timer lets you. It's those attempts that make you gain, after all... There's no point in waiting until your next stealth check comes around "naturally" (that'll be quite some time at the higher levels - the amount of steps you can take depends on your skill, it's ten steps at GM).
Lastly, you're best off training in your house (where you will never fail to hide). You can't attempt to stealth unless you successfully hide, so training where hiding can fail is only limiting your check rate (and hence your gain rate).
