I used to use a tomato-based sauce, but a couple of years ago I moved to my version of a mustard-vinegar sauce. This involves cooking my pork shoulders in or over an oven dish, as I feel the juices are too valuable to go down the drain. While the pork is resting, I pour off the grease, and deglaze the pan with cider vinegar - it is amazing the amount of piggy goodness that you get out of the sticky mess. Then I thicken with mustard powder, stir until completely smooth, then add either English (alcoholic) cider or apple juice until the sauce is the right consistency
Pull the pork, mix in the sauce, and then let it spend the night (at least) in the fridge, to come together, before reheating and serving.
This probably all sounds like heresy, but it results in great tasting pork, and no worry about when it will be ready.