This part is actually quite nontrivial, I found. I have a text file full of headlines, or copywriting, even though we're not quite at the point to use them just yet. It's mostly for us; I add to the list constantly, attempting to sell the project to myself every time, as if I was a user. When it seems too hard to describe it in such a way, it's a flag suggesting we might want to refocus.
Also check out http://copyblogger.com, The Ultimate Sales Letter by Dan Kennedy and Triggers by Joe Sugarman.