I tried a spreadsheet, but since I have two accounts with different use years I came up with a different system. I have a word document that I call "My DVC Journal" and it has been easy and fun to do. I keep track of my points for a three year period so that I can track my points and keep all of the banking and borrowing straight. I do this for each of my accounts and each time I book a trip and/or take a trip I make notes to myself, like a journal, and update my three year points chart. I just started this system this year and it works great. I also think that it will be cool to read back in the journal say five years from now. This also gives me detailed notes about what I do with my points so I don't get confused, and I actually caught a 50 point MS mistake on my account (which I got back, no problem)
Think about what memories you can record and read back in twenty or thirty years. Whatever system you use, try to keep it as simple as possible, or you could lose points with a mistake.