1. Use good cheese
2. Use good bread
3. Grate the cheese so it will melt better
4. Grill it on a heavy pan, like cast iron, at a fairly low heat. It's amazing how quickly it can go from golden to black.
So without further ado, here's how I made my most recent sandwich:
First, spread something interesting on one side of a slice of good whole-grain bread. In this case, I used my homemade pesto. You can find the recipe at http://morning-glory-garden.blogspot.com/2009/10/pesto-and-salad-dressing.html . Then sprinkle on about 1/3 of the grated cheese you'll use for each sandwich. Layers of melted cheese help hold the other layers together. Here I used a nice extra-sharp cheddar.
Then add a layer of tomato slices (or crisp, thin apple slices, but not with pesto - that sounds weird).
http://morning-glory-garden.blogspot.com/2009/10/soup-of-evening-beautiful-soup.html) and some fruit. All those lovely flavors held together by the cheese are a real sensuous treat.