It looks like open policies vary widely.
One idea that I saw at several campuses for limiting risk management issues (this will depend on the size of the party of course as to whether your chapter could do this):
We would take the keys from anyone that drove to the party, and issue basically a coat check ticket for their keys. If they overindulged (I know, THAT doesn't happen

), we wouldn't give the keys back. We'd either offer to drive their car home for them, or give them a ride, and they could get the car keys back the next day. Driving their car made me nervous due to liability issues, but we figured those issues were nowhere near what the liability of letting them drive.
We also used wristbands (legal age, underage, and designated drivers) to determine who could drink what, and, if any $ was associated, DD's were free anyways...