This posture sensor was developed for the class Rest of You at ITP. It uses a wearable Lilypad Arduino, flex sensor, and wearable vibration motor sewn into a tight-fitted shirt.
I've been working with devices to tackle poor posture, and the idea of a subliminal reminder that you have bad posture while working on a computer was the motivation behind this project.
I tested a flex sensor in various positions in my back, using an LED to indicate when my posture was bad. I found that the small of my back was the place with the biggest bend. I set a threshold value on the Lilypad and caused the vibration motor to trigger if the flex sensor passed this value.