I just throw whatever sounds good together for a dry rub. You can't go wrong with paprika, garlic powder, onion powder, black pepper and dried thyme. If I'm doing Mexican, I'll add cumin and chili powder.
You do have to be careful not to burn the rub, though. Otherwise, you'll have to peel off the crust and hope the meat is cooked inside, or you'll have to continue cooking it without the rub, which renders the whole idea pointless.