[thesimp]

Evernote. But don't get obsessed by technical details of what tool is best: Evernote, Onenote, Tiddlywiki, etc... Just make sure you have all work/private/short term/long term notes in one place. Preferably accessible via multiple devices so that you can always access them.

And depending on the note type go through them every day/week/month to see if they are still relevant. Some people like to go the whole GTD way but I like to pick just the parts that work for me.