GO Airport Express offers airport shuttle services between Chicago area locations and O'Hare and Midway Airports. Efficient, safe, and economical airport shuttles depart from O'Hare and Midway Airports every 15 minutes for Chicago downtown and many suburban locations. Ground transportation is arranged by making a reservation. Please Click Here to make your airport shuttle reservation/s, or call 1-800-284-3826 and use your 10% discount code: TwiFic (all one word).
If you're looking to share a shuttle ride with someone to save even more, the lovely La Geologia created a spreadsheet to hook up with people arriving/departing around the same time.