We usually do a park, sleep, fly. I get nervous about getting to BWI from PA on time (we get morning flights for the most part) and have had luck with trinityreservations.com We stayed at a Country Inn and Suites within a couple miles of the airport for just over $100 including tax last fall through that website. That particular deal let us park up to 14 days inclusive. Considering the price of parking elsewhere, the hotel stay was nearly free for that night.
Otherwise, we have just parked at the express lot at BWI because it's so convenient when coming home tired from Disney. I haven't parked elsewhere.