I'm a huge cheerleader for ParkExpress, best garage going. However, coming from O'Fallon you would end up overshooting the airport to get to it, as it's located at 170 @ Natural Bridge. I can't say enough good things about the service, though -- vans every 3 minutes, choice of valet, indoor, and surface lots, complimentary drinks when you arrive, emergency service if you have a flat or a dead battery. Just a wonderfully run business, and the rates are decent, too; $7.95/day if you stay over a Saturday night.
The website is
http://www.parkexpress-stl.com/
Over holidays the surface lot sometimes fills up, but the garage almost never does. If the lot is full when you arrive, they will open the gate and let you park in the garage anyway, using your surface lot ticket, which gets you surface lot rates.