taken from Thad Curtz's comment in the Prius Co-op? thread

At first I thought there wasn't much to tell - we just did it... But that's not quite right.

• We did it with people we've known for thirty years.
• They live about seven houses down the street from us.
• We've shared a number of other things with them over the years. We shared a lot of child care with them when our kids were little. We split a CSA share with them for a few years. We've co-owned a freezer with them for years - it's in their garage, so it's easy to get to. We've owned a several lawn mowers with them (one after another, not all at once). We're working (really slowly) on putting a mesh wireless network on our block to share a high speed Internet connection with them and other people.
• They own another car, so there's a backup when two people want to drive at once. There's also somebody else on the block with another car that it would be easy for us to borrow, if we ever had to.
• When we started this, my wife was retired and I was riding the bus to work. One of our friends was retired and one was car-pooling to Shelton, so she drove the Prius three days a week, but we knew in advance when it would be gone. Now I'm retired, and the one of us who's still working usually takes the bus. Bottom line - the four of us have probably been driving less than many people do.
• Sometimes, we schedule ahead - when somebody's taking the Prius off on a trip, or when they want to use it for some regular, repeating event. For a stretch, one of our friends used it to go visit his mother in Lacey every Wednesday morning. Usually, we just call up a little ahead of time or at the last minute; I actually can't remember a single time when I've wanted to hop in the car and do an errand and it's been a problem, but having a backup car helps insure against that.

We bought the car second-hand, and split the price ($5,000 or so each). We keep a little book in the side pocket and each time you get in the car you enter the mileage before you drive off, if you're switching from one household to the other. (If your household drove it the last time it was driven, you just keep driving.) When you put gas in, or take it to the garage, you enter what you spent in the book. One of us enters this stuff in a spreadsheet, and we split all the expenses according to how much each household has driven.

Problems?

• As I said a while back, it took a fair amount of calling around to find an insurance company that didn't say "Share a car? Why would you want to do that?" Progressive insures us by having one person in each household on the policy and one person in each household listed as additional drivers. We haven't had any claims. They only give the primary drivers little insurance cards, not the additional drivers; I wish I had a little card too. We carry 250K, 250K, 500K and comprehensive and collision with a $500 deductible - the premium's $695 a year.
• At one point, we let the car go for considerably longer than I wish we had without getting the oil changed because I was assuming that our friends (who used to drive rally races and are much more interested in cars than I am) were taking care of getting it serviced, and actually none of us were paying attention to it. But it only happened once - now I'm explicitly in charge of making sure the oil gets changed on time.

It works fine for us - but as you can see, a number of things help it make it easy which aren't part of every possible car share.