Pretty much what CAD said, except I do inject. Probably not necessary, but I follow a recipe that has served me well many times.

You can't screw up pork butt too much unless you pull it too early. It can take forever sometimes to get thru the stall (around 160-170) and get on up to 190+, where it needs to be. If you pull too early you may have to serve sliced rather than pulled pork.

I use the recipe below. It is for a whole shoulder (never see those here), but the amounts of rub and injection work well for two butts, which is what I do. I skip the saucing step. Serve sauce on the side for those who want it.

Chris Lilly's Six-time World Championship Pork Shoulder

Recipe By : Chris Lilly of Big Bob Gibson's

For the pork shoulder rub
1/4 cup dark brown sugar
1/2 cup white sugar
1/2 cup paprika
1/3 cup garlic salt
1/3 cup kosher salt
1 tablespoon chili powder
1 teaspoon oregano leaves
1 teaspoon cayenne pepper
1 teaspoon ground cumin
1 teaspoon black pepper
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Pork injection
3/4 cup apple juice
1/2 cup water
1/2 cup sugar
1/4 cup salt
2 tablespoons Worcestershire
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1 whole pork shoulder (approximately 16 pounds)
1 bottle Big Bob Gibson Championship Red Sauce (or substitute your favorite BBQ sauce)

Inject pork shoulder evenly with injection solution.

Apply a generous amount of rub onto meat. Pat so the rub will adhere.

Place in a smoker and cook with indirect heat for 16 hours on 225°F.

Serve with sauce on the side or paint shoulder with sauce the last 20 minutes of cooking.

When done, the pork should pull off the bones easily. The internal temperature of the pork should reach 195°F.

Plenty of good info at the Virtual Weber Bullet under cooking topics and on the bulletin board. Here is a whole page on pork butt issues.