Mid-2011, I started tracking my exhibition and graphic design work. I work for a number of clients so I track by project, the duration of the project, the payment intervals for each project, and when multiple projects overlap.
I track everything; from year-long projects to those that might last just a few weeks. It's not scientific and I'm not sure what I'm accomplishing by this, but as a visual person and as a freelancer, it's been very helpful for me too look at my year as a whole.
And if nothing else, I have nice little graphic to chart my work, right?
p.s. I am the first person to admit that I might like Excel a little too much.