Here is how I do mine. I start two nights before I plan to smoke:
1. Inject the butt with a brine solution (learned this while watching the BBQ nationals):
In a 2 Cup measuring cup:
1/4 C salt
1/4 C sugar
Equal amounts water and apple cider vinegar to bring level to 2 cups. Stir until salt and sugar are dissolved and inject into the meat with a Cajun injector and refrigerate until the next evening.
Note: I've started adding about 5 drops of BBQ bitters to this marinade, really gives a good flavor to the pork:
http://gardenandgun.com/article/how-make-your-own-bbq-bitters
2. The next evening, rub the butt with your favorite rub (I use Salt Lick, a rub sold here in Texas). Refrigerate over night.
3. Next morning, get your smoker ready: heat to about 250-275.
Wrap the butt tightly in foil and cook for an hour per pound. Unwrap carefully and save the juices for basting (will be almost a quart - use leftover for making beans, etc). Get a very smokey fire going and smoke the butt at about 200-225 for one hour, basting with the juices every 15 minutes. This pork will be so tender that the bread you serve it on will be tough in comparison.