Do you ever cave in and allow your kids something just so you can use it as leverage later? It has to be something that is OK for them to have, at least in small amounts or once in a while. For example, I might let the kiddo have a treat at school just so I can claim the last cookie. Or give her the go-ahead to watch TV so that I can claim my right to the same privilege later in the day. I usually phrase it like "well, you did x earlier, so I get to do x now". I could just go ahead and eat the cookie when she is in school or record a TV show and watch it when she is sleeping or at school, but my goal with this approach is to teach her about fairness, and that Mommy deserves a treat once in a while, too.
(Sometimes, though, I treat the kiddo and myself at the same time--like on Monday, we went out for tea/hot chocolate.)