1. make sure boot fit is proper in the binding, DIN is set precisely on both front and back and the binding and anti-friction pad are clean and lubed lightly.
2. On the carpet put your strong side in the ski, have a friend/wife etc. sta nd on the tails and do the following: With a bent/flexed knee go forward as hard as you can. Your heel should release/not to hard/not to easy.
3. On the carpet, with knee bent and flexed, twist your foot to the inside to release the toe, if you can barely do this then that is about right (unless your are a real weenie and couch potatoe etc.)
Some of the above comments are right on the money and a few are really off. Basically you should come out on a real bad fall even if it is just one ski and not on "easy" falls. the leg doer in falls are the very slow twisters/try not to put yourself into these predicaments. In answer to your question in 35 plus years and a lot of falls: Premature releases; about 6, one racing (chattered a ski off in a SG), couple times in heavy wet manky powder and a couple other times. I weigh 193 and run a DIN of 8 and find that works for me.
As one of the above posters said they won't work all the time but you can narrow the error portion down by being aware of your skiing abilities and your bindings workings.