I keep track of my ideas in a Moleskine Notebook. They are available in many formats and are able to whitstand almost anything. What I love about having a small "book" of ideas is that I can take them anywhere, scrible and brainstorm around my ideas etc. All this is possible without being restricted by a given format. Its also very easy to browse through ideas if you store them this way and I found it very easy to find ideas again once you remember you had some solution or approach to something.
If I by chance don't have my Moleskine with me I email myself the idea and as soon as I get home or start emptying my mailbox the idea gets transfered into the notebook.