During Summer of Code 2005 and 2006, we reviewed hundreds of applications, from which 25 students were selected to participate. This year we are likely to receive even more. Competition will be fierce, so what can you do to help make sure your application gets serious consideration? Here are some tips, straight from last year's mentors themselves.
Sell your idea. Describe your idea in detail. What is its ultimate goal? What components will it have? What benefits does it have for Drupal itself and its community? How do you plan to achieve completion of your project? If a specification already exists, what will you do that will go above and beyond expectations?
Sell yourself. Get across your enthusiasm for the project. Tell us what makes you stand out from the rest of the crowd. Talk about your past experiences, what makes you tick. Why are you interested in open source software, and Drupal in particular? What interests do you have, and how do these interests relate to the project for which you're applying? There is a basic assumption that people applying for Summer of Code will have at least some programming skills already. So rather than spend a lot of time elaborating on these (though by all means, do tell us what you know), spend time talking about you.