Object Orientated Design Patterns.
Conforming to true design patterns is really a little more involved than some may think.
If you do not have the resources to get your hands on some of the fantastic books out there here is a link to Principles of OOD. This is a very good start for anyone who wants to get a good foundation.
http://www.butunclebob.com/ArticleS.UncleBob.PrinciplesOfObjectOrientedDesign