There are a variety of ways in which to promote ebooks, digital downloads, books, and memberships. I'm going to list a few here to give you an idea of the plethora of ways you can begin promoting Muscle-Build.com products.

• Review the product. You're going to be promoting affiliate products predominantly through the web, right? People use search engines to search for information and they could use your site for their research. Write up an objective review, giving the benefits, drawbacks, and other details about the product.
• Use the product. Tell your readers how the product worked for you. Include "before" and "after" photos, journal entries, and other notes. This makes your "testimonial" much more relevant and believable. People want unbiased information. Give it to them.
• Blog about an aspect about the product and include an affiliate link in your post.
• Write an article about the product and submit it to the article directories like ezinearticles.com, GoArticles.com, and SearchWarp.com. Put a link back to your site sharing other relevant material about the product OR direct link to the vendor's site (read the Terms of Service at the various article directories – they all have different rules about this.
• Post a classified ad. Believe it or not, people buy digital downloads through Craigslist and other classified sites. Put up an ad and either drive traffic to your site or directly to the product's sales page (with your aff link, of course).
• Pay Per Click – run an ad on Google's Adwords or Yahoo or Bing. They're just like classified ads except you may pay per click or per impression. Be wary of this method – it's highly effective but you can lose a LOT of money if you're not careful.
• Advertise in other people's ezines and newsletters. Find like-minded individuals and see if they'll work with you.
• Purchase banner and text ads on other sites. This is very effective.
• Guest blog for other publishers with a link back to your site. There, you will promote your affiliate link.

These are just a few of the many ways to promote affiliate products. I suggest you try one or two, tweak them a bit, and evaluate your results.