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First, find a crock pot big enough for your ham. Ours was huge, but only because we had to fit 7 pounds of spiral-sliced goodness into it :) Put the ham in the crock pot with whatever juices it's been packed in. In a bowl, mix two cups brown sugar with one cup water. Pour into the crock pot. Add a can of diced pineapple, with all the juice. (Adjust the amount to the size of your ham. The next time we do this, I'll use a much smaller cut. I'll probably halve the recipe.)
If you want to fancy-up the presentation, stud the outside of the ham with pineapple rings instead of just pouring them in. Use toothpicks or cloves to "pin" the rings in place. I did not have time or inclination for this after wrestling a 20-pound turkey into -- and out of -- a basting bag.
We cooked the thing all night on the "warm" setting. Put it on low if you're aiming for it to be done when you get home from work. Put it on high if you've only got 4 hours or so. Since the ham is usually already cooked, you really just have to get it hot all the way through.