The problem with steak leftovers is that they never taste as good if you simply reheat them.
The solution is, clearly, steak quesadillas.
Start with cold steak and a sharp, unserrated blade. Trim off any fat (which should be easy to spot, since it'll be white). Figure out which way the grain of the meat is going, and start cutting perpendicular to it in order to make the meat more tender. Slice as thinly as you're able, and cut the slices into 1 inch lengths. Lightly salt.
Use real flour tortillas. If you see a sign advertising handmade tortillas, buy them. Otherwise, resort to buying from the market: look for something locally made with the least number of ingredients. I'm a fan of Alejandro's, which you can purchase at Walmart. Get the large size.
Heat two tortillas in a large, ungreased skillet on a medium-low setting, turning them quickly so they're warm but not starting to crisp. Put the least crispy side face down and place your steak on the bottom tortilla, making sure all slices are flat. Pile on finely shredded cheese (I prefer the whites: mozzerella, jack, Parmesan and Romano), making sure enough fits between the beef to make it stay together (the mortar to your bricks, if you will). Press the second tortilla on top.
Cook until the cheese is at least half-melted and the bottom is just starting to get crispy, and then flip. Before removing from the heat, ensure that all the cheese is completely gooey. Slice with your knife from before, and serve piping hot, accompanied by steak sauce.
The better your original cut of meat, the better the result. Steak quesadillas in restaurants often use inferior, chewy steak, and it makes a huge and delicious difference when you make it at home and pay attention to the details.