Never sell your product - always sell service. Any obvious sales language in a sales letter will create a corresponding amount of purchase resistance. These are the wise words of Claude Hopkins in his books My Life in Advertising and Scientific Advertising. These books were written in the mid 1920's but the principals are still valid today.

In the real world, this is a free tasting of cheese at the grocery store, or a coupon for a free can of beans. In the Internet world, it's a free match.com account - you don't pay until it's proven to you that it's worth it. It could also be a free trial period for some software. It could be online banking. It could be a content site that provides information, and makes money on hosting PPC ads.

If you're selling something on the Internet - information in particular - give a little away to get a lot in return. Build some confidence and credibility with the customer and they'll reward you with a purchase.

For example, if you're selling an e-book, give away a top 10 list (or a white paper, or a taste of your software) in exchange for an email address. You build cred with the customer, and get the ability to send a couple of email marketing messages to them.