The most important parts of a sales letter are the headline, sub-headings, bullet points, your guarantee and P.S.
A headline will get the reader's attention. The sub-headings should read like an offer. The prospect should be able to read the sub-headings and be presented with a summary of the offer.
Every sales letter should have bullet points. These should all be the benefits of your offer. An iron-clad guarantee will put the prospect at ease and finally the P.S. re-states your offer. Many prospects will read the headline, skim the sub-headings and bullets, then continue on to the price and P.S..
The most important thing to remember when writing a sales letter is benefits. Benefits can make or break an offer. The prospect is not interested in you or the features of your product. They want to know what the product will do for them.
A great resource that I liked is, Maria, Veloso's, "Web Copy That Sells" which can be found on Amazon.