I've made a lot of granny square blankets. I usually have a baby blanket going for charity, and I make a twin size blanket for each of my kids. Since I always make these blankets with multiple colors, I like to spend some time mapping out where each color is going to go. Not only does it keep me organized, but it helps me plan out how many skeins I will need of each color.
In the past, I've always mapped out colors on paper. Unfortunately, paper is easy to lose or tear, and with small children around, my plans often fall victim to small hands. One of the benefits of digital technology in my life has been the ability to read and write things without worrying about interference from a local toddler.
The last couple blankets I've worked on I have mapped out in Google Sheets. While it's time consuming to use the color fill function in each individual square of a blanket, I like mapping out a fairly realistic representation of what I plan to make.