How to create rolling backups for SQL Server 2008 R2
Ola Hallengren's excellent scripts provide solutions for database maintenance including backups - the scripts have won some awards in the last year, and are increasingly referenced by SQL Server luminaries. I've been using them for about a year - I love them because they are highly configurable - for backups, you define the retention period in the job in hours. The retention period only kicks in if you actually have refresh backups.

http://ola.hallengren.com/