Preselling is the work you must start doing before you release and sell your product; that's why it's called pre-selling. It's the work you do to convince your to-be-customers that your product has great benefit and only a fool will pass your offer without purchasing -- all this without actually "selling" the mentioned product.
Preselling can be seen as selling yourself to the customer before you sell your product. When you sell yourself to the customer, you are making your customers think "okay, this guy is not one of those greedy marketers. He's actually here to help me out." so that they will let their guard down and listen to your sales pitch.
Preselling is also the art of building up anticipation. If a friend told you his chronic headache has been solved overnight by “some new medicine” but did not mention what it really was, would that leave you wondering what the medicine really is? More so if you're having a chronic headache yourself! That's why by mentioning a benefit which would get your prospects' ears perked up again and again but not really revealing what the solution is, you will get your prospects dying to hear what you have to say next as you reveal slightly more about the solution each time. When you finally uncover the whole thing, your prospects will be crawling all over your website looking for the buy button!
However, preselling is not just about unveiling your product bit by bit. In fact, that was only one of the various ways one can presell. For example, you might run a weekly newsletter on acne problems and coincidentally you have just written this great ebook called “10 Ways to Solve Acne Woes Once and For All”. In your newsletter, you can presell by including a snippet or two from your ebook and mentioning it as a viable solution to acne problems. If people get good advice from your newsletter, they will perceive you as an expert on acne problems and naturally will be curious to find out the ways you can teach them to solve their problems.
At the end of the day, it all boils down to giving value to people before you ask people to buy something from you. If you can help people to solve their problems, they will, more often than not, be very willing to buy products from you.
