I would suggest parking at either Franconia-Springfeld, Huntington, Whirle-reston East or Greenbelt (depending where you are coming from) and taking the Metro to Reagan. The parking fills up at those stations quickly and has a daily cost of $8.95 per day during Monday-Friday. DC Metro Parking is free during weekends. It's easiest from Franconia-Springfield or Huntington because they are on lines that go directly to DCA.