It is recommended that you rotate your crops, or plant your crops in different locations each season. In order to do that, I have made a map of my small and crowded garden each season.
Some people are blessed with good memory for small details. I am not one of those people. I knew that I would never remember what was planted where in seasons past, unless I wrote it down.
My system works for me, so I will share it with you.
I have a spiral notebook that I use in the garden and garage. I contains lists and ideas. It has plans for garden trellises, maps of our sprinkler systems, ideas for fences, etc.
I find that having one notebook keeps me organized (as long as I can find the notebook). It is beat up, dirty and priceless. It has been the keeper of my ideas and plans for several years. Looking through it I realize how busy I have been. Many projects are represented there. Including my garden crop maps.
My garden map is roughly to scale and I write down what is planted, and the date it was planted. If I planted seeds I write an "s" next to the date. If the plant was transplanted I just write the date.
This year I have also started a garden map of harvest dates and yields. I am being inconsistent about keeping this map updated to be honest. My vegetable garden is in its second summer season, and last year I kept no records of problems or yields, and I wish I had.
In the notebook I have also been writing down some notes about my crops including pest problems, remedies I've tried, quality of the crop, etc. Gardening is a learning process and this information will be valuable for next season. I guess all that note taking experience I got it school is paying off, my notes are essential in the garden, my new classroom.