High level projects are years. I started with Aperture in 2009, so being conventional, I have 2 high level Projects, 2009 and 2010.
Under each year I create projects by event with a date. Example 2010-04-11 Mark's Wedding. I'm English so the format is YYYY-MM-DD. If I have several days its YYYY-MM-DD_DD_DD
Now I have found that I want to split out some stuff for easier uploading to SmugMug, so I have two other 'types' of folders underneath the year. One is for Surfers (this summer I did about 200 shots per day of surfers), the other is for a sport called Pelote Basque (which Jai Alai is part of).
Basically the reasoning is that the date stuff is usually with my family and so goes into galleries that are shared by grandparents etc, and the other stuff is for non-family 'events'.
I don't know if its the best way to organise, but its my way.