What I am about to tell is way too easy. Get beef boullion or broth, put it in a pan and bring it to a boil. Lay the prime rib in this for a few minutes. You will not lose the tenderness or flavor....
very gently! Cover the salmon with a damp cloth to retain moisture, and heat slowly in a low oven. If you use a stove top skillet, turn the heat down very low in order to let the salmon heat evenly...
The answer depends entirely on the sort of leftover to be reheated. Leftover pizza would be reheated quite differently than leftover pot roast or soup.