I've bug tested on a few mods before, and I find that the number 1 rule for it is to have a notepad and pen beside you. then jot down EVERY little bug you find. You may be surprised, I personally found that I was remembering 3 - 4 times as many bugs. when you look at the bug list at the end of the session you will be amazed.
This is good advice for you mappers out there too. Note them down, and when you fix them cross them off.