I second Evernote. While I don't use it for this particular problem, I use it for TONS of other things. I can't even tell you how many apps this single app has replaced for me.
I third Evernote. I keep a single note for each largish task or smallish project and add points for each new thing I solve. It's great to come back to 6 months down the track...I remember doing this before, but how did I fix it then?