I use light wheat bread (a store brand, 40 cals per slice), and lowfat or part-skim cheeses. I'll use a little bit of real butter or olive oil usually... I'm not a big fan of cooking sprays and I don't buy butter substitutes because I use butter so rarely.
I try to always pair it with something healthy, like homemade vegetable soup. Yum.
My favorite grilled cheese EVER is with mozzarella, thinly sliced tomato, and fresh basil. Now I'm hungry.