We pretty much always fly out of BWI since it's a whole lot cheaper then our home airport. We started using Park N Fly to park. They are cheaper then the long term parking lot. It's a secure lot that is fenced in and the shuttle bus picks you up near your car and drops you off at the airport. We have never had much of a wait for the bus back to the parking lot. You pay when you leave. They give you a ticket when you go in to park. They usually have a coupon online for a discounted rate. They usually offer water on the bus too!