Following is a link that seems to provide solid advice, and is clearly written, but only regarding backups to snapshots: http://www.techrepublic.com/blog/datacenter/backing-up-and-restoring-snapshots-on-amazon-ec2-machines/5434
This method is a bit risky if Amazon has a complete failure, so additionally, would want to use a method such as rsync to do traditional OS backups to an offsite machine. Such as rsync (which I will cover in a subsequent post).
I only use snapshots to record major events in the system lifetime, like initial configuration. Which is good because you don't want a lot of snapshots, anyway. OS backups using rsync are done twice daily from cron.
To restore a failed system, I would create a new instance from a snapshot, then restore from my latest rsync backup.