Knowing how to cook a perfect steak is a worthwhile skill to have. But if you have leftovers and want to reheat them without drying them out, this method from America's Test Kitchen is perfect.
Essentially, the trick for reheating cooked steak without losing moisture or that flavourful crust is to duplicate the cooking method that got it to where it was in the first place. Start it off slow in the oven, then sear it in a pan, the way the video above describes. I've been known to use this cooking method and the reverse (searing first and then finishing in the oven) when making steaks, but I'll admit this technique probably works better to retain moisture in already-cooked steaks.
While "leftover steak" isn't exactly a problem I think most people are likely to have, it is a great way to properly reheat leftovers from a restaurant or a big backyard grilling session.