The creation and subsequent enforcement of a new rule is part of the glee of being a Dom. There have been some very good ideas presented in this thread; I would urge you to listen. Particularly in cases wherein you both lead hectic lives, a written contract adds a certain permanancy, clarity, and focus to the aspects of a relationship you and your dom choose to specify. X, Y, or Z may be a whim or pretext created on the spur-of-the-moment simply for the Dom's immediate pleasure, be it enjoying your obedience or administering your punishment. If your Dom intends to make a new rule, perhaps have him utilize certain phraseology; if he doesn't use that exact term you may consider it an order of short duration.

If you do not have a contract, consider making one and adding a clause whereby it specifies that unless an order is written into the document, it shall be existant for the day (or other length of time specified) and automatically expire or be expunged thereafter. This would likely soothe your mind, for then you are obeying, and your Dom is carrying out his end. If you need more consistency, it is your duty to bring said fact to your Dom's attention.

DeDomMaster