As a long time user of ZipCar/Flexcar (I originally joined Flexcar in 2004 and was absorbed into ZipCar when they merged), I have a few tips I can share with others. If you have some tips you've learned, please add them to the comments.
14 minutes - Sometimes you need a car for just slightly longer than an hour. Zipcar allows you to take the car up to 14 minutes early without additional charge. If you take it 15 minutes early, though, you'll get charged for an extra 1/2 hour. I will often reserve a car that is unreserved for the immediately preceding time period, knowing that I can then take it for an extra 14 minutes.
I've never tested the grace period on the back end, because the fine for a late return is $50 (see next tip). I think I've gotten away with a minute or two, but I've never tried to stretch it more than that.
Text messaging and extensions - Sometimes the problem is on the back end, you're not sure you can make it back on time. You can always reserve for an extra 1/2 hour, but then if you return it more than 30 minutes before the end of the reservation, you feel slightly cheated.
Here's what to do. Sign up for text alerts (under the "help" tab, select Zipcar mobile). Your car will text you about 20-30 minutes before your reservation ends. You can text back (not while driving, though!!) right up to the very last second. Suppose traffic is tricky and you're not sure you are going to make it in time, but you might OR say you are picking someone up at the airport, and you don't know how long you'll have to wait for them to come out. You can wait right up until :59 and then send the extension txt message; your reservation will be instantly extended.
Of course, this only works if the car is not reserved after you, Which brings us to. . .
Before and after strategies - If you are going to use either of the preceding tips, then you'll want to manage your reservation timing. Perfect would be a car that had a 1/2 hour gap on both ends, because no one can reserve it in those gaps. Then you would know it was available 14 minutes early and also available for a last-second extension if needed. Longer gaps will work, too, since other drivers aren't likely to try to squeeze a reservation in between two other drivers--especially if there are other cars available.
In the scenario above, I can pick up the car as early as 11:16 for no additional charge. I can then keep it until 12:30. As late as 12:29 I can txt for an extension and keep the car until 1:00 if needed, obviously being charged for the additional 30 minutes if I request the extension. This gives me the flexibility to use the car for up to 1:44 with an original 1-hour reservation.
Walk around the car first - Speaking from personal experience, make sure you look for damage before you get in. I had a long, painful battle with Zipcar over pre-existing damage, because I did not report it until a few minutes into my reservation.
Retrieving forgotten items - If you leave something in the car, you can retrieve it for up to three hours without charge. Your ZipCard will still unlock the vehicle. If you don't drive anywhere, then there is no additional charge. Hopefully the car is still there when you realize you left your phone in it.
Steve Offutt lives car-lite with help from Zipcar