GSoC 2008 participant here. arjunnarayan is correct, you're not applying to Google. I hung around the forums for months leading up to the application process. I had also created a crude prototype already. I mocked up some screenshots in GIMP of what I envisioned the finished product, which I think helped a lot.
Be passionate about the group (Twitter), as well as your project. Answer questions people have about your proposal.
Be passionate about the group (Twitter), as well as your project. Answer questions people have about your proposal.