I just read a great article in MSDN Magazine (available online here) on the Open / Closed Principle from the Patterns in Practice column.  I really enjoyed it.  I'm going to have to spend some more time learning some more design patterns.  I've spent some time learning Model-View-Presenter (MVP) and Inversion of Control (IoC) but haven't put either into practice yet.  Who knows what the future holds though?