There seems to be a lot of questions on how to set up dyndns with EC2. It is fairly easy to do but I haven't seen anyone put everything together to do it yet so I figured I would write a little example using ZoneEdit. I picked ZoneEdit because it lets you sign up and host 5 domains for free.
First you need to sign up for a free ZoneEdit account and add your domain. I'll assume you can do this. Don't forget to change your domain to point to the ZoneEdit DNS servers.