The pro's keep in mind what is called the "100 Degrees of Doom." That is the 100 degrees between 40° and 140° F. This means that all foods served cold should be below 40° F. and all foods served hot should be heated to above 140° F. The 100° in between is the perfect temperature for bacteria to grow in food. Of course, this doesn't happen immediately, so the rule of thumb is that you don't leave food out for over 2 hours.
When reheating leftovers, bring sauces, soups, and gravies to a boil. Heat other leftovers thoroughly to 165° F. Microwave leftovers using lids or vented plastic wrap for thorough heating.