I highly recommend reading the "Java Design patterns 101" tutorial, and its follow-up "Java Design patterns 201" , if you are unfamiliar with the concept of Design Patterns (and see below for more information). Note that you have to register at the IBM DeveloperWorks site in order to read these, but that's a good idea anyway.

You can find more information about patterns from the following links:

1. Brian Foote's Pattern documents
2. "Java Design patterns 101" at DeveloperWorks
3. "Java Design patterns 201" at DeveloperWorks
4. "Java Design patterns: Elements of Reusable Object-Oriented Software" by the GoF (printed book)