I've never had a cast before and am getting one. It's for a break in the base of my thumb so it could be over the wrist or entire thumb. Is there anything I should do in preparation (it's such a small fracture I can in fact use my hand)? I hear casts can get scratchy, is there anything I can do before putting the cast on that would prevent this? It's on my dominant hand.
Giving the area a wash using an exfoliant to get off as much dead skin as possible, but without damaging the skin, then applying a perfume free emollient such as Diprobase will help initially, but may not be possible because the area is too painful with whatever problem you've got that requires a cast. Itching beneath casts is caused because of the presence of the cast - dead skin cells can't be washed off nor fall off because they're trapped inside the cast, they build up, and your skin starts to itch because the body wants you to abrade the area to remove the build up. I'm afraid it's something up with which you must put, to coin a phrase - using a hairdryer set on cool and aiming it between the cast and your skin (if there's a little gap after a while) might ease the itching for a short period. Some anti histamines will reduce the itching, if you can take those.
As above, exfoliate as much as you can without pain, as it's your hand, I won't tell you not to shave it as I assume you wouldn't anyway.
Just like the other guys tell you, exfoliate. Also, go get baby powder. Usually it gets kinda hot and sweaty depending on the type of cast used. Keeping it dry in there will keep it from itching and smelling bad.