A pretty good store bought rub I used last weekend was called "Applewood rub". It came in a round container. Pretty good stuff. Spread yellow mustard all over that baby and rub that rub on there, wrap in foil and put in fridge over night. Take out about 30 minutes before firing up the smoker and put more rub on there and leave uncovered so it gets closer to room temp. Get smoker between 225-250 degrees. I add wood chips the first 6-8 hours then wrap in foil for the last 2-3 hours. Total cook time depends, i use a thermometer and when it hits 190 degrees i take it out, wrap in foil, and put in an empty cooler. Usually takes me anywhere between 10-14 hours. Let sit for about 45 min - hour then pull it apart with your hands and enjoy. Nothing like that first bite of the bark to see how it came out!!!
Read the comment below about injecting and it reminded me, mix some water, apple cider vinegar, and that rub in a cup and inject away in the boston butt. Good luck! Take pictures so the rest of us can salivate.