Expressing mild annoyance
I am looking for the mildest of curses, something like "Rats - I left out the は again!"
imiwa? gives チェ but since I have no feeling for nuance in Japanese yet, I thought I'd better check.
チェ is less a word and more a general sound of frustration. In my experience it's used mostly by men, and often when something isn't working properly or things aren't going their way. It can also be used to express disbelief or skepticism for something, in a haughty sort of way.
もう! is probably closer to what you're looking for. It's very mild -- not even really a curse -- and people use it a lot both for general problems and for their own mistakes. It's a bit like "geez!" or "darn it!" Ex:
もう！みせはもうしまった！ = Darn! The store already closed!
もう！ またわすれた！ = Darn! I forgot again!
もうぅぅぅぅ! = (general noise of whining/complaint)