Begin by using a nonstick pan with a quick spray of olive oil rather than slathering bread with loads of butter. That is really going to lighten the load.
Be choosey about cheese. The amount of fat per ounce can vary greatly. Goat and feta both lower fat options. If you use a mere two ounces, you don't have to break your calorie bank because melting cheese will make it go further.
Next, go with a sassy low-cal sauce. Think about condiments that will add zing to your sammie. Salsa, mustard, olive tapenade or even a dollop of Trader Joe's bruschetta are all great choices.
Now go with grain. Thinly-sliced sandwich thins are a nice choice because they are whole grain and keep the fiber up and calories down. A whole wheat tortilla makes a grill cheese a quesadilla - with only one slice. Look for bread that is less than 90 calories each.
Your sandwich can become a masterpiece with the add-ons. Produce that is cruciferous, crunchy and filled with fluid will help fill you up and add bulk to the mix. Consider apple slices, mushrooms, spinach or the combo. Of course, tomato is also a good choice.
And if by chance you want more protein, think about rinsed can tuna or a few slices of deli meat to help keep you fuller longer.
Making an open-faced sandwich can save you about 100 calories while still enjoying that melty goodness.