There are a couple brands of spiral cut hams that are ok, but they're generally pretty sad.
That said, basically what you want to do is glaze it. There are a million and one ham glazes, but here's a very basic one to get you started
1 cup dark brown sugar
1/2 cup maple syrup (Or honey, if you don't have maple syrup... It's a little cheaper)
2 tablespoons brandy, rum, or bourbon whiskey(as you prefer)
2-4 tablespoons brown mustard... to taste, it's to cut the sweet a bit
1 tablespoon soy sauce
1 teaspoon chili powder
1/2 teaspoon crushed red pepper flakes
Combine this all in a sauce pan, simmer for 15 minutes or so, stirring frequently. Warm the ham as instructed, brushing with glaze before it goes in the oven. When the ham is "done" according to the instructions, raise the oven temp to 400 or so, brush again with the glaze, cook for 10 minutes, repeat until glaze is gone or the ham looks delicious.