When I make a grilled cheese I typically use a good multi-grain bread (My favorite is Franz Bakeries San Juan Wheat Nine Grain Bread) then use multiple cheeses. I make a layer of each for nice, thick, gooey sandwiches. Chedder, american, then monteray jack.
The american melts the fastest, so I always put that in the center so the harder cheeses are a little closer to the heat.
Now here's the real secret! I butter the outsides of the bread, then sprinkle them with a little bit of garlic and onion powder. Then I actually grill my grilled cheese on my BBQ as opposed to using a frying a pan.
Get the grill good and hot, then cook them over indirect heat to keep the bread from getting burned. It takes a fair amount of time for all of that cheese to melt all the way through. It will be dripping out the sides long before the very center of the sandwich is all melty!
Sorry to bring up an old thread, but I just saw this and figured that everyone loves a good, simple grilled cheese recipe! Also, if you enjoy yours with tomato soup much like myself, I've found that you can make a can of Campbells tomato soup taste nice and classy by just adding a dash of basil to the soup as you heat it up!