Take a bread that makes a good toast (to you).

Take a cheese that is soft, mellow (unless you really love pungent) and melts well under heat.

Combine such that the bread ends up perfectly toasted and the cheese in the middle melted at the same time.

I sometimes wonder if a sufficiently buttered initial toast side would make for a good inner face for the cheese to go against such that the sandwich might hold up for that much longer.

PREVENT the sandwich from sitting face down on any surface and eat while still quite warm. When I still at these frequently I'd either do it myself or ask for the sandwich to be plated such that the exposed surface area was maximized to allow the steam to escape and prevent soggy sandwiches.