I use Preflight Parking, some type of operation as the other(s) mentioned. You can print a discount coupon off their web site, and also make a reservation if you're the paranoid type who doesn't want to risk finding the "full" sign out when you arrive.

Like the other off-site locations, they follow you to your parking row in the shuttle on arrival. I'm usually at the terminal within 10 minutes of arriving at the lot.

http://www.preflightparking.com/BWI.aspx