So you locked your key in the car. Wire hangers is how it seems to work in movies, but in reality a wire hanger just doesn't offer enough leverage. This is a step by step guide that should work with cars that have vertical button locks.

[edit] Steps

Obtain umbrella wires- the perfect item for the job.
- - You should have 1 or 2 of the LONGER wire ribs (parts that spring out to open umbrellas). You should also have a hand tool (e.g. pliers) with a rubber handle, to pry open the car door just a tad.
Crack the window open from the top, using a wire. You will encounter resistance from the weather stripping. Push this out of the way using whatever it is that you used to crack the window open. You will be able to affix it back to the car frame when you are done.
Use a rubber handle of your hand tool to pry the window open, the width of a set of pliers is pretty perfect.
Break an umbrella. Remove 1 of 2 of the longer ribs and strip of any glue. You want just the rib (rod). Test the selected rod for leverage. The top end (i.e. the end that connected to the shaft of the umbrella) might even be a bit flat, this will help you. Your rod should offer enough leverage (push) against your hand while held by the other end.
With your car door still pried open using the rubber handle of a hand tool, slip the umbrella rod (or 2, if you intertwined a pair) through the crack at the top of the window, and push down on the vertical button, most likely located at the top of the door handle.