I’ve been tracking projects on a spreadsheet for awhile in order to make sure I’m on time/budget, but I also recently started using a timeline application to help visualize the scheduling, and that’s been a huge improvement. You get a much better picture of what’s going on at any given point when you’re looking at it visually, and that’s a big help when trying to space projects out so that there aren’t too many or too few at once. So far I’ve been using TimeGlider and am pretty happy with it.
Update 9/20/2012: I switched to Timeline a few months ago and like it even more than TimeGlider. It’s a desktop app, so it’s much more responsive.