At one time, it was possible to wait in line each morning at the FL to get a reservation two months from that day. We were able to get into FL the first time because the concierge at our hotel (the Vintage Inn) did that for us (very nice tip for her!). Unfortunately I don't think that FL takes walk-in reservations anymore. That being said, here are two alternatives:
Book a table for 8 or more up to one year in advance. They also take walk-ins the same day if you show up with 8 people. Somehow they always manage to find room for such a large group without a reservation. Thomas Keller himself suggested that option when he was here in DC on the Bouchon cookbook tour.
Another option is to get on the waiting list as far in advance as possible, for each day that you'll be in Napa. Make actual reservations at other restaurants just in case, but while in Napa, call FL in the early afternoon to see if they had any cancellations for that night. Our second trip to FL happened using this method. We were on the waiting list each of the 4 nights we were there and phoned once each day around 1pm to check. On the 4th day, they called back with an opening for that evening. Made that early flight the next morning from Oakland a bit difficult, but it was definitely worthwhile.
Other awesome restaurants in Napa --
St. Helena: Must go to Martini House! Rustic interior with an open kitchen in back. BIG food so be very hungry. The chef LOVES mushrooms and even offers a complete mushroom tasting menu. The cream of mushroom soup is fabulous. Great wine bar downstairs that also serves many of the same menu items. http://www.martinihouse.com/
Terra, also in St. Helena, is another excellent place. Very fresh ingredients with an emphasis on fish. http://www.terrarestaurant.com/
Calistoga: roadside barbeque joint -- I have no idea what the name is -- noted for tri-tip and pork with a sauce so hot that the employees there won't even try it! Lots of habaneros-- I recommend ordering a milkshake with that one. They do offer another less-painful sauce too.