I've been keeping a garden journal for the past few years. The goal of this journal is to make notes (both good and bad) on things I have done and observed in my garden. Every year, I can return to the journals from previous years, so I can use this information to enhance how I go about setting up my garden for the current growing cycle.
The most important part is that it helps me to avoid making mistakes again and again. As any gardener knows, sometimes mistakes happen, but the key is learning to not repeat these mistakes. Having this journal has really enhanced my gardening experience.