In previous posts I've explained how to use a hex editor to corrupt a database for testing and exploration. MVP Tony Rogerson has gone one step further by using bulk load to import a database into SQL Server and then create various corruptions using regular T-SQL. Checkout his *fantastic* blog post
and look through some of my earlier posts for details of on-disk data structures that you can corrupt.
Its very important that you try running CHECKDB on a known corrupt database so you can test whatever logic you have to notify you of database corruptions.