The first think to decide is the size you need. Just get the same size heated mattress pad as the bed that it will go on.
Power source. The best heated mattress pads, such as the Sunbeam convert the 120 coming out of the wall into a safe 14-18 volts. Choose a heated mattress pad with this feature, you can never skimp on safety. Also make sure the power cord is long enough.
Wiring. The power unit should be easily detachable from the pad, allowing you to put the heated mattress pad into the washer for easy cleaning.
Heating elements. Today's heated mattress pads use heating elements of very fine wire woven directly into the fabric, called micro-wire, these micro-wires are practically undetectable. Look for this feature as it also ensures no hot or cold spots. This is an important feature of heated mattress pads when it comes to the ultimate comfort level.
Material. As the heated mattress pad will be below you, under a sheet the material is not as important as it is for an electric blanket. Nonetheless, to get the best heated mattress pad out there, look for good quality polyester/cotton material.
The combination of quality material and micro-wiring will allow most high quality heated mattress pads to be machine washable and tumbled dry without shrinkage or damage. Make sure the heated mattress pad you are considering specifically says machine washable.
An additional feature to look for are many power level settings so you can set your heated mattress pad to exactly the temperature you like.
For safety, the heated mattress pad should have auto-off after 10 hours or so. Another nice feature to have is a quick warmup setting.
A great feature to have, especially for queen or king size heated mattress pads is a separate temperature control for each side of the bed. Some like it hot, some don't, avoid conflict!