Alerting on Database Mirroring Events
Joe Sack's answer on ServerFault is as good as it gets:

https://serverfault.com/questions/251551/sql-server-2008-mirroring-threshold-alerts-at-database-level/251989#251989

For some background, Joe runs the Microsoft Certified Master of SQL Server program.  He knows his stuff.  (I can vouch for him personally.)