It depends partially on what the punishment is for. You can get an idea of some examples through searching threads here, they should give you a few ideas.
Well, I would suggest using anything you know they particularly hate against them. For me anyhow, orgasm denial is my least favorite, so it tend to be a punishment of choice.
You're not providing enough information for us to make any suggestions. Is the sub male or female? Just a sub or a slave? One-off disobedience or something systematic? Is the sub making a sincere effort or being bratty? What does the sub like and dislike? We're not psychic.