My wife and I don't own a car, and we rarely miss having one, partly because we belong to City Car Share. When we decide we need a car, once a month or so (for visiting friends farther away, or to do some heavy duty grocery shopping), we reserve one on City Car Share's website a day or two in advance. There are a bunch of cars within a 10 minutes walk of our apartment. When we're done we return the car to its lot (no city-parking headaches), and head home. City Car Share is a non-profit, but there are a couple for-profit competitors: ZipCar and FlexCar.
Post your own solution: www.sierraclub.org/solutions/