Read the mattress pad label. Some will say to machine wash in cold water while some will say warm water. Some will say tumble dry low and still others will recommend not to dry in the dryer. Some will say to dry clean only and others might specify no bleach. This may not tell you much or how to wash it exactly, but you can use this as a guide of what not to do.
Use a duvet cover. If your mattress pad is made of down feathers, memory foam, egg crate foam or wool, you may want to invest in a duvet cover. You can always wash a duvet cover from general wear and keep your special pad protected.
Dry your mattress pad completely. Your label may say you can tumble dry low, however, this could still shrink it. An alternative would be to hang it dry and if you do so in the sun, this will help kill bacteria and dust mites. Be sure the pad is dried completely before you put it back on your bed so you don't create an environment where mold and mildew may grow.
Spot clean your mattress pad. If you have a wool, down filled, memory foam or an egg crate mattress pad, you will need to spot clean. Even if it has a duvet cover, spills and accidents that seep into the pad do happen. If this is the case, wash the duvet cover if you have it and then soak up any liquid with towel or vacuum up (with wet vac) the pad itself. Then mix a little Woolite or gentle detergent with water and gently spray or dab it on the soiled spots. If it's a urine stain, then use an enzyme cleaner available at most pet stores. Then soak up or vacuum again and repeat the process with water. Allow the pad to air dry completely, preferable in the sun, before putting it back on the bed. If the label says you can dry clean, you may wish to do this instead for urine or other severe accidents.
Clean your plastic mattress pad. Some pads have a plastic side to prevent your mattress from getting wet. Clean the plastic side with a wet rag with soap or a baby wipe that is alcohol free (alcohol may ruin the plastic). You can also use a disinfectant spray or wipe to kill off any bacteria or if the pad was soiled. If the pad has a soft side to it, do not put it in the washer because the plastic may get torn up. Instead, wash by hand with mild detergent and water and hang up to dry, preferably in the sun.