As a person that uses computers all day to do things, you would think that I'd be on board with the idea of using Sketchup or the likes to plan my garage. I tried that for a while, but I was having a tough time coming up with a layout that would work. I ended up using my computer to create outlines of all of my things, which I then printed and cut out. I also printed out a scale outline of my garage. I found that I could try different things out much quicker when they were little bits of paper I could move around on a whim, rather than (click, drag, rotate, drag, click, rotate, drag, drag, rotate, drag, rotate). Call me old fashioned, but I've been clicking and dragging on a computer since I was 3, and I find that it can hinder the creative process sometimes.
The only side effect of this process is that my dog ate my metal lathe. I managed to get the milling machine out of his mouth before he managed to chew it up too much.