Effect:

You pick up a random spectator as you walk down the street. You ask them to come with you to a nearby car and pick a dead fly off the windscreen. You hold it in your hand do a few passes with the other hand and appear to be really concentrating and the fly slowly comes back to life to the surprise of the spectator.

Preparation:

Catch a fly and freeze it. You can immobilize the fly by swatting it hard enough. Place it in a small container in the freezer. After the fly is frozen, quickly take it to a suitable windscreen IN THE SHADE and place it there. It is important that the car is in the shade to prevent the fly from defrosting too quickly.

Procedure:

Get a random passerby (near the car) and bring him to the car which appears to be randomly picked. The heat of your hand will defrost the fly which should start moving as soon as that happens.

Freezing the fly with dryice or any of those aerosol canned air products for cleaning computers work better as they freeze the fly quicker and cause less damage.