Here's an instructable detailing plans for an AVR based device which helps you realize you're dreaming -
What is Lucid Dreaming? Lucid Dreaming is when you are sleeping and dreaming and that you are aware that your are asleep and that you are dreaming, yet you don't wake up. This can be great, giving you full control of your dreams, jumping off of buildings, going into space, whatever your mind can create.While I'm not sure "jumping off of buildings" is my top pick for a lucid dreaming activity - this project definitely seems worth a try. - The Lucid Dream Machine
The Lucid Dream Machine is a pair of glasses that you wear while you are sleeping. About 4 hours into your sleep the AVR microcontroller pulse LEDs that shine through your eyelids. This half wakes you up. The flashing lights helps you become aware (in your sleep) that your are sleeping and dreaming, in doing so you become more likely to be able to control your dreams.
As a simpler alternative, you could try wearing a digital watch and train yourself to always check it twice. Our brains have difficulty reproducing the same digits twice - upon second glance in a dream the numbers usually appear incorrect, thus clueing you in on your current state of consciousness. I've used this technique myself with good results.