Get the right bread. Almost any bread can work for a grilled cheese, but start out with bread that complements the remaining ingredients and doesn’t have too many (large) holes. If you’re going for a sweet chèvre and jam grilled cheese, go for a few slices of country bread.
Step outside of your cheese comfort zone. Not that there’s anything wrong with slices of American cheese product, but sample different options like Havarti, Brie, and even manchego.
Choose cheese that will melt. Hard cheese (Parmigiano-Reggiano and the like), most fresh crumbly cheese (feta), and other kinds of cheese (like halloumi) just refuse to melt. Pro tip: grated cheese melts faster and more evenly than slices of cheese.
Stuff it. Grilled cheese can become a meal if you stuff it with some vegetables and a protein (I’m looking at you, Cubano sandwiches). Be sure to take this into account when you’re choosing bread; you’ll need something hearty enough to handle the weight.
Low and slow. When you’re craving a grilled cheese, you don’t necessarily want to waste time, but if the pan is too hot it will burn the outside of the bread without melting the cheese properly. Melt some butter in the pan and put the sandwich in when the butter starts to bubble. Don’t move the sandwich around and wait until the bottom is golden brown to flip.