As RaceyB put it, marinading is typically left to the smaller, leaner cuts of meat, but they can be used on larger cuts as well, more time will be needed in the marinade for the butts and briskets. Typically I will inject a butt or brisket the night before, then pull it out of the fridge right before I start my fire in my pit, then I rub it down and let sit out until my smoker is at temp. For briskets I prefer to inject with a commercial injection, either Butchers BBQ or Kosmos. On pork butts, I like Chris Lilly's injection mix: 3/4 cup apple juice, 1/2 cup water, 1/2 cup white sugar, 1/4 cup table salt, 2 TBSP worcestershire. Mix, simmer until the solids are dissolved, then chill prior to injecting. Best tip is to make the injection a few days prior to using it for the flavors to blend.