If you're feeling lucky you can also park across the street from the bike shop near Java Hut. It's a permit-only lot but so far I haven't been ticketed.
For those of you who don't know about this trail, it's a 12 mile mostly-singletrack route on land owned by the boy scouts. To ride here, you must obtain a permit which you can purchase for $5 at the Sunshine bike shop in Fairfax, or online at the 'Boy Scouts of Marin' website.
Lets meet up in Fairfax (in front of the bike shop) at just before 10 - whoever needs to purchase a permit can do so then, there's a kiosk in the shop where you can fill out the application and get your tag.
The trail itself I consider challenging; lots of switchbacks, tree-roots, steep ascents, with some rocky downhill spots and a few blind corners. Beautiful though!
Here's the route: http://www.mapmyride.com/routes/view/149695519