We usually stay the night before our flight at a hotel in Essington (usually around $80). We get one nights accomodations plus up to 2 weeks parking. You just leave your car there and take the hotel shuttle to the airport. This has worked great for us. We have stayed at the Holiday Inn, Comfort Inn and Ramada. The Ramada also has a secured lot that you can park in. I don't remember the exact price, but it wasn't bad.
If you do consider staying overnight though, the Ramada was gross and I wouldn't stay there again if it was free.