Emails work well because of the 'Echo Effect.' Basically meaning: You are reminded of something without having to do anything. Think of the product 'Head-On'. Though their commercials are annoying, you will never forget them...in fact, you probably are finding yourself saying/thinking it right now... Head-On Apply Directly To Your ForeHead

The beauty of the Echo Effect is that people are constantly reminded and people eventually think of your product/concept. So even if they just go to your website once, they will more likely be repeaters as long as they are given incentives to remember.