This is one of the reasons that you'll often see sweepstakes void in certain states. If this is your first time sponsoring a sweepstakes, make sure you check with local laws about possible restrictions. Local marketing and promotion agencies can help you with this information. Entering sweepstakes has been a viable hobby for many years. The way you handle your promotion will show people whether you are legitimate or not.
Legitimacy is the key.
Many sites hold a sweepstakes just for the sole purpose of gathering names. For example, one site chose to give away a custom designed T-shirt every month as the grand prize. Create a nice graphic or order form with the discount posted right on it. Don't hide it or disguise it as a sweepstakes. If you hold a sweepstakes just for the sole purpose of gathering names, you are quickly going make a lot of enemies on the Web.