Reheating grilled hamburger patties
If you have a pre-cooked burger patty, I'd suggest heating it in a frying pan / skillet over a high heat, add a splash of water and then cover with a small heat-proof bowl (a cloche), or a lid on your pan. The water will steam the burger back into life, adding moisture and trapping the heat to more thoroughly warm it through. 

I also use this technique when initially cooking burgers if you're not grilling outside - does wonders to melt any cheese you lay over the top of the patty in the pan!