Google Summer of Code Checklist

Pre-requisites: Except having a good grasp in one programming language, you need to have the knowledge of git/svn/mercurial/bzr. The reason is that all open source communities use one of these tools to manage their code. Collaborative development would have been very difficult otherwise...

Start early
I didn't. Well, in a way I did, but the organization didn't make it through! Anyways, the best time to start contributions is the window of December-February. In December, you probably would have your winter vacations. Take full advantage of that. Another advantage of starting early is that you get to know the community beforehand, something which would be useful during the application sorting process.

Join the mailing list
Every community has a public developer mailing list for discussions. Join it. Be careful regarding one thing though. Some communities have a separate one for the GSoC (like Honeynet or Drupal). Make sure you join that as well. Also, this is the place where you are supposed to introduce yourself.

irc
IRC stands for Internet Relay Chat. Think of it as just a chat room. Be active in the irc. Make people remember your name.

Read the documentation
Although new projects don't have documentation, but read it before attempting to solve issues in their code. Make sure whatever you read reflects in your code too.

Submit a patch
Very crucial. In fact, the most crucial part. If your organization has a code base, it is absolutely necessary to submit a patch to prove to the community that you understand their code.

Making the proposal
This single document would make or break your GSoC. Make sure you add plenty of discussion points and flowcharts. Proposals usually don't have a word limit, and I think you should make it as long as you can. The longer it is, the greater your changes.