Adding moisture is the best way to stop rice from becoming dry and hard during reheating. There are several ways to do this, depending on how you plan to reheat the rice. There is some concern over the safety of reheating rice, but generally problem bacteria grows only if cooked rice is improperly cooled and stored -- not during the reheating process.
Rice can carry the bacillus cereus bacteria, a known cause of food poisoning. Unlike many food-borne bacteria, though, bacillus cereus can withstand high temperatures, so reheating the rice alone will not kill the bacteria. The best way to avoid food poisoning with rice is to store it properly. After cooking, cool the rice quickly and thoroughly before refrigerating. Never leave rice sitting out for more than an hour and a half. Do not store cooked rice for more than a few days.
The fastest way to reheat rice is in the microwave. Cover the rice with a wet paper towel and cook for one minute. Check the temperature of the rice and cook longer if necessary. If the paper towel has dried out, remoisten it before restarting the microwave. You can also sprinkle the cooked rice with water and cover with a microwave dinner plate or plastic cover to hold in moisture.
Steaming rice will give it a moist, fluffy texture that more closely resembles fresh rice. You can steam your leftover rice in a steamer. If you don't have one, put the rice in a colander and place the colander over a pan of boiling water. Stir the rice occasionally to get an even temperature.
Reheating rice dishes
Reheating a dish that contains rice is easier than reheating plain rice. The extra ingredients will give the rice moisture. That said, you'll still want to add some liquid. If you are reheating the dish in the microwave, you can use the same technique of placing a wet paper towel over the bowl of leftovers. If you're reheating on the stove, add water or broth to the food to give it needed moisture while reheating.