I'm a big fan of Wally Park. They're the only long term parking company that has hanging pads between the cars to prevent those that have parked next to you from dinging up your car. Their shuttles travel round the clock and they pick you up and drop you off right at your car's location.
They have AAA discounts. If you travel enough, join their Wally Club which gives you a free car wash, bottled water and access to their copier and fax machine.

My parents really like, and often use, the Fly Away in the San Fernando Valley that was already mentioned. It's not close to the airport but it's cheap.