A text file? Google doc? Spreadsheet? What questions do you ask when you write down a new idea?
Edit: Thanks guys, I didn't expect to get that many responses. Some interesting approaches in here :) Personally I put them into regular text documents. For each of my ideas I try to answer the six product-focused YC application questions as a sanity check.
The good ideas -- the ones worth doing -- keep coming back to you, gnawing at you until you can't put them off any longer. It's a real problem you continually run into, and it's something you care about. So much more valuable than a list of half-baked ideas you had randomly on the bus.