hey jon. i replied to your post in "roll call" before i saw this one. have you read this thread? http://www.smokingmeatforums.com/viewtopic.php?t=1639
there is some good info there. i make my mop out of apple juice, cayenne pepper, garlic powder, ground black pepper, onion powder, and brown sugar. sometimes i use chili powder instead of the cayenne pepper, depends on if my kids are here or not. as for when to mop, read the "basic brisket smoke" thread i linked above. it is a sticky at the top of this section.
I spray every meat that I smoke (except meatloaf) with a mix of 3 parts apple juice and 1 part bourbon. I spray every 45 mins to an hour, when I need to add wood chunks to keep up the smoke. The sugars in the mix carmelize, turn brown, and add to the "bark" on the outside of the meat, especially pork butts and briskets.