Make sure that the gifts that you are giving is worth the time and effort for people to join the contest. If people do not think that the gifts are worth the effort, they will not even join in and do the contest.
Make it so that all can join in if they choose.
Make it optional for them to join in or not.
Give them ideas on how they can do better with the contest from time to time and keep them up to date with what is happening in the contest.
Make it fun and easy to do.
Have clear guidelines on the way to win: Example--if you do this amount of sales or higher then you can win this prize.
Make sure that you name the winner so that every one know who it was.
Make sure that the gifts that you are giving is worth the time and effort for people to join the contest. If people do not think that the gifts are worth the effort, they will not even join in and do the contest.
Make it so that all can join in if they choose.
Make it optional for them to join in or not.
Give them ideas on how they can do better with the contest from time to time and keep them up to date with what is happening in the contest.