SF2G Preparation Suggestions

bike

air: put at least 100 psi (road) or 60 psi (mtb/hybrid) into your tires, and you'll prevent most flats.
tires: if more than 4 years old, very difficult to remove/replace, or visibly haggard, replace them.
tubes: bring at least one extra, and make sure the size is right and the valve is long enough; also practice changing a tire at least once (how-to video).
pump/CO2: bring a small pump or CO2 cartridge and suitable head/valve for it.
once-over: make sure your bike works ok and has reasonable lube on the chain, brakes don't rub, quick-release skewers are fairly tight, etc. If your bike makes sufficient noise during normal operation to obviate the need for a bell, you need to get it fixed.
seat: make sure the height is right, and consider buying a newer style saddle with anatomical design for your gender... and try it on a shorter ride first!
map: print out a map and throw it in your back pocket in case you get separated from the group. (maps on the group page, see link in nav.)
food & water: bring at least one water bottle and one source of calories (such as beer or a powerbar-like item).

fitness: be sure you can handle a 40 mile ride -- if you aren't very experienced, this will mean about 3 hours on a bike seat, so 20-30 mile practice rides are a good idea. try the Millbway variant of the Bayway ride before trying the full Monty -- it's about 20 miles and flat.
gear: invest in basic cycling wear, especially shorts with a chamois (padding). they don't just look good, they make your life much more comfortable (at least physically).
shoes/cleats: flat pedals are fine for around town, but you'll be happier on 40-mile rides with shoes that clip-in to your pedals. not mandatory, just advisable.
attitude: this is a fairly challenging ride if you're new to the sport, so bring a sunny disposition and patience, and you'll have a great time.

that's about it! come out and try it! you'll dig it.