re: Just put 4 pork butts on the smoker...

I too have problems with meat drying out from time to time. Sometimes you get a bad cut of meat, there was nothing you could do it was going to be tough and dry. Briskets are notorious for this.

Some basics I use...

I used to trim fat from the cuts. Now I do little if any. I find I have more moist flavorful meat this way. However, you do sacrifice the pretty smoke ring.

I always use a mustard slather. Mine is based with yellow mustard, beer, hot sauce, and kosher salt. It keeps the meats surface moist and allows the rub to adhere to the meat. This works especially well with pork.

Rubs vary from one smoke to the next. There are lots of blends out there. I find black pepper turns bitter, so I steer clear of it.

I always have my meat thawed and at room temperature when it hits the pit. I have a Lang offset smoker, and I cook at 200-225 degrees.

A good thermometer is a must, overcooking meat will leave it dry and tough.

Lastly, I like to keep the meat moist in the later phase of the smoke by spritzing it with an apple juice/whiskey blend. I do this roughly every hour after the meats surface is dry and a crust forms. Usually about half way through a smoke I find.