We also have used Safety Jackpot for Two years now. We are in the 2nd phaze which is the Gold tickets. You can set up your own criteria for the program. We have a 10K jackpot drawing annually based on the size of our warehouse. That doesn't mean the person who grabs the bags wins 10K, our first person won 500$. However, they can win 10K if they draw the two bags with the 10K voucher in them.
Our Criteria is similar to the other person who responded:
No Preventable accidents, No absentieesim or tardiness, etc. We hand out 1 card per week, and a bonus card for the month. It is based on individualism, so you aren't relying on weather your whole department goes without an incident. You have opportunities to earn extra cards though with contests, going ABCD, etc.
I strongly feel that there needs to be a complete safety program with toolbox talks, safety committee, etc. This alone will not reduce you injuries. In fact, our stretching program we just rolled out in September has reduced our injuries far more significantly than the Jackpot program. But it's just like anything else. You have to continually talk about it and "sell" it to the employees for it to be effective.