The phrase, "you can't have your cake and eat it" has had me a bit worried for a long time. I thought it meant that you shouldn't eat cake, or that "real men don't eat cake" or something. I've just googled the meaning, and what a daft thing it is to say. If you eat your cake, you are "having" it, and if you've got enough, no doubt you don't even want any more, so why say you can't have it?
Another saying I've come across is, "never eat the last piece of cake". I think that one makes sense, though it has to be balanced with the principal of "waste not want not".