Taken from http://www.bearshistory.com/tailgating/index.aspx:
"The following are things to know if you'd like to tailgate before a Bears game:
The only way to be guaranteed a parking spot will be available whenever you'd like to arrive is if you have a parking pass for the South Lot, Waldron Deck or North Garage. And the only way to get a pass is to be a season ticket holder and win a lottery in the offseason. But yet, there is still a catch with the Bears--the lottery you would apply for does not differentiate between tailgating lots (South, Waldron) and non-tailgating lots (North Garage). So even if one were to win this lottery, they may be stuck paying for non-tailgating passes.
So that leaves cash parking for anyone without a pass, which includes our group. In 2004, the Bears banned canopies, open fires and deep fryers from the lots, so in order to get one of the extremely limited number of grass spots where you are able to use a canopy to block wind, plan on getting there before 7 a.m.
Cash parking lots are at the Field Museum, the Adler Planetarium, Burnham Harbor and 31st Street. The current cost (2005) to park a car is $35, RVs and buses are $75. Be prepared to change plans on the fly, because the Bears/city/General Parking change the availablity of lots each week without notifying fans in any way. Meaning if you parked in Burnham Harbor one week and liked it, you may plan on parking there with a group the next week and find it blocked until 10 a.m.
Good luck if you can navigate all the Bears' restrictions and still have a good time. We always do, after shedding the stress of getting things set up."