I do not need to convince any copywriter how powerful even a single bonus can be to improve the conversion rates on your sales letters. If you sell a $97 product, and visitors are not convinced the product is worth $97, you could throw in some extra materials that have been proven to be worth $47 or $97 to make the buying process a very simple decision. Do you have trouble thinking up the perfect bonus to add to your offer? Do you have too many or too few bonuses, and you do not know how to apply them? You need to decide if you want to offer standard, surprise, loyal, or last-ditch bonuses.
Standard bonuses are those bonuses you offer directly on your sales letter and use them to sell your product. All bonuses, especially these standard ones, need to be directly related to your original offer. The only problem with standard bonuses is that it is possible to have too many bonuses. For that reason, I tend to only stick four or less bonuses on my regular sales letter, sometimes none, because adding too many bonuses draws away from the value of your regular product.