As far as what to look out for, just make sure you don't over cook them and dry them out or cook them to the point of completely falling apart. Either baste them to add moisture, or foil them towards the end of the cook with whatever liquid you want (might I recommend some Sweet Baby Ray's). That adds tons of moisture and tenderness but exact specifics on how long and how much are really up to you. You have to experiment with it to find the right combination. The 3-2-1 method is a good starting point, but shouldn't be relied on 100%. My spares recipe when I'm cooking for the immediate family follow that method but mine is more of a 2-2-1. I think a 6 hour cook with 1 or 2 racks of spares over cooks them. In my early stages of figuring out my recipe, the 6 hours always made the ribs come out just a little over cooked. I'd go to pull them off the grate and they would fall apart. The best ribs stay together and come "clean off the bone" when eaten, not "fall off the bone".
Just have to experiment and figure out what works best with your brand of ribs, your cooker, and your cooking style. In the end, ribs are hard to really mess up so the results are usually pretty good regardless.