Summarize this article:

When you take a bath or shower, put plastic around the cast and don't put that part of your body into the water. You can use plastic bags or trash bags secured with waterproof tape to help keep your arm dry. When you are taking a bath, you must not get your cast wet even if you have a fiberglass cast. Don't try to wash the cast in the shower. It may be easier to give children a sponge bath to ensure that their cast stays dry. If your fiberglass cast has a waterproof liner, you might be able to take a shower. Ask your doctor before you try. If you happen to get your plaster cast wet, or you sweat inside your cast, you need to dry it inside and out. To do this, use a hairdryer set to the cool setting. This will provide air that will dry any moisture inside and outside of the cast. Don't use the hairdryer set on the warm or hot setting. This can burn the skin and cause problems. It can also cause more sweating and moisture in the cast. Your fiberglass cast with a waterproof liner can get wet, either in the shower or swimming pool. After you get out of the water, it will take around an hour for the inside of the cast to dry. Be patient and let the cast dry naturally. Don't use a hairdryer to try to dry the fiberglass cast faster. Don't stick a towel in or around the fiberglass cast.

Summary:
Protect the cast when you bathe or shower. Dry your plaster cast with a hairdryer on cool. Let a fiberglass cast dry naturally.