I have been using Park Ride Fly USA for years. They are about two miles north of Midway and have a secure lot. I have never had any issues leaving my car there. When you enter the shuttle will meet you where you park then take you to departures. When you return from your trip you just call them and they will pick you up and drop you at your car. This is so much nicer than waiting for a random shuttle like you will find at other lots. If you sign up at their website they usually will send 10% off coupons and they do have a frequent user program to earn free days.
The cost is $7.25 + tax for uncovered parking or $11.25 + tax for covered.
http://www.parkrideflyusa.com/mdw-chicago-midway-airport-parking/
Hope that helps. Have a great trip!