I'm going to take the train....I don't know if there is one from O'Hare, but from Downtown there is a train (I'm sure you can get to this line somehow, someway from O'Hare)...the above poster gave us pretty accurate directions (based on waht I had looked up in preparation). It will take a long time, but I'd suggest doing it to save on the trip up (if yo have time). I booked my "hotel" at Days Inn for like 60 bucks for the one night that I'll stay there..it's only 1.0 miles away from RFU, much more convenient and cheaper than the recommended accomodations.
Splurge on the return trip--taxi and whatnot from RFU to O'Hare, since you will likely have to "rush" to catch a return flight...take a leisurely train ride up to RFU from the airport since there is less of a rush, and probably not much to do in the surrounding area.