Cheddar, Swiss, and Provolone cheeses all taste great in a ham sandwich.
Don't microwave mayonnaise.
Adding other types of meat to your "ham" sandwich could make it taste better.
Try putting your entire finished sandwich in the toaster oven for just a minute or two to melt the cheese.
The clerk at your local deli can slice the ham as thin or as thick as you like. Experiment with several thin slices or a few thick slices to see which tastes best to you.
Variation: Substitute pastrami for the meat and rye for the bread.