"What is the trick to getting a perfect loop? It seems that when I play it is not obvious when to push the stop button."
One of these days I need to get around to making a video about looping. Until then all I can tell you is that you have to hit the pedal the second time at the exact same point you began. So for example, if you start at beat one of a measure, hit the pedal precisely on that beat both the first and second times. Also, play through after you hit the pedal the second time as if you were going to keep going. Otherwise you might slow down or stop at the end of the loop in a manner that makes the transition back to the beginning noticeable. You want it to sound seamless.
I use a Boss RC20 Loop Station in my guitar theory DVDs.
Play Until Yer Fingers Bleed!
Mr. Desi Serna
Podcast: Search Desi Serna at iTunes