What are all the elements that a landing page should consist of? I know that there should be a buy button somewhere, some testimonials... what else.
You need to have a comparison between your product and other product. Other than that,you should have tutorial on how to use your product.
I think you need to give a little more info. Are you talking about a sales page for a list or a page you will be using for first time visitors? If you are looking for first time visitors, I'd say a basic squeeze page is best. As far as sales pages go, mine are extremely simple. I try to 'sell' as much as I can through aweber instead of making a sales page thats 8 miles long.