Whats even harder is attempting to warn drivers when you are going to turn. You could throw your hand out, but this can make you loose stability and most people don't remember the ways to put your hands. This instructable seeks to make warning drivers and other cyclists that much more easier, and make your more visible while doing it especially at night.
Alas this is the premise for the Bicycle Turn Indicators, another pal in the "Make Your Bike Into A Car Series"
Step 1: The Flashing Circuit
A note about flashing circuit relays for cars, There are two kinds electrical and mechanic. Make sure you pick up an Electrical one, i made this mistake, and got the right one off ebay. Below are some pictures.