In my quest to become an ever better decision maker, I began keeping track of my decisions a while back. Now I am building up a nice stack of notecards to review, and it is too much. So I recently moved them to a tickler GTD style (see the picture).
It works like this… when I make a major decision about something I write down the key points, assumptions, and reasoning behind the decision on a notecard. I use different colors for work-related and personal stuff. I decide at what point in the future I would like to review the decision. If I want to review it in three months, I just drop it slot for November. If it is a longer term thing and I want to review it in a year or two, I put the notecard in an envelope with the year on the front, and I put the envelope in the appropriate month. I only keep 1-2 envelopes per month, so I add multiple cards to them if I need to.
When the time comes, my tickler reminds me of my decisions and I can look to see how accurate my assumptions were, faults in my reasoning, etc. Then I can move the card(s) to a new slot or envelope. That allows me to review a long-term decision at 3 months, 6 months, and then once a year. I've only recently implemented this, but it works well so far on a month-to-month basis. I assume it will work well over the long-term too.
Decision making is an important business skill, and like any other skill, it should be analyzed so that you can improve. Don't rely on your own memory, because it has a way of filtering your decisions in ways that are skewed by your current views. The decision cards only take a few minutes to fill out (and I only do them for major decisions, or decisions that are very confusing) and are extremely educational if you actually review them. A word of warning though, they can also be very depressing when you realize how many bonehead things you've done over the years.