I’ve tried a couple Aperture folder hierarchies and this is the one that seems to work the best for me.
Initially I imported my entire image collection into one library with subfolders for the date. I quickly realized that I was going to hit the 10,000 image-per-project limit. I’ve now decided on a different organization that has worked really well. The basic idea is major category folders at the top, year folders under that and all projects below that. For example:
Depending on the shoot I take between 30 and 500 images. Each shoot goes into a single project which is placed in the correct major category and year. I take a lot of photographs and so far this has made it really easy to locate the images I need. Now, I’m not a fancy-pants professional, so your mileage may vary.