Q: How do I reheat left-over grilled meat without ruining it?
A: Slow and gentle is a start.
There's no getting around the fact that reheated meat will never be quite as moist as when it first came off the grill, but you can take steps to mitigate dryness.
Wrapped in foil in a 250° oven is a good way to go—use a meat thermometer, and keep the meat below its maximum temp from the day it was cooked, around 130° for a medium-rare steak or 165° for a chicken breast. Don't slice until after reheating. Also, a microwave on low power is a surprisingly effective tool—just ditch the foil.
Adding moisture in the form of sauces can help. If you use a marinade, set some aside (before it touches the raw meat) and use it as a quick moisturizing sauce the day after, stirring in any juices from the cutting board.
You could also take a cue from fancy chefs and modern sous-vide techniques: Place your meat in a zip-top bag with the air squeezed out of it, and submerge it in a pot filled with water at around 130° (the temperature of your average hot tap). In 30 to 45 minutes, your meat should be warmed through and ready to serve. If you want to add a quick crust to the exterior, pat the meat dry with paper towels, and sear it ever so briefly in a bit of oil heated in a ripping hot skillet.
But the best way? Just slice up that steak or chicken and serve it with some chilled grilled asparagus in a light vinaigrette, or layer it with cheddar cheese to make a pressed panini.
Kenji Lopez-Alt is the chief creative officer of Serious Eats (seriouseats.com), where he writes The Food Lab, unraveling the science of home cooking.