A principle in Time Management For System Administrators is that we shouldn't trust our brain to remember things. It's better to write something down, or record it somehow, rather than trust our brain. Besides, it leaves more brain space for important things.
I often recommend that if you don't have a way to write something down, call and leave yourself voicemail. There is a company called Jott.com that does this even better. You call them, say what you want, and the text of what you said is emailed to you, along with a link to the audio file. They use caller-id to determine who you are. The voice-recognition is pretty good so far. (I've used it twice). They keep an archive of what you've recorded. The UI when you dial in is very good (I like the fact that it is safe to hang up after you hear they've received your message.)
You can also "jott" to other users.
It's now an open beta. Sign up here.