This might be obvious but I just discovered it today: If you have one of the newer two-finger-scrolling trackpads, you can keep one finger anywhere on the trackpad while moving only one finger to scroll.
[robg adds: Probably obvious to some, but worth sharing if you haven't realized you don't need to drag both fingers...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20061123011822357