I just finished reading the book Head First Design Patterns. If you always struggled around with design patterns this is definitely the best point to start.
The book is covering all details for becoming familiar with the idea of design patterns in short time. It is comprehensive, easy to understand and suprisingly also entertaining and fun to read at the same time which you won’t usually except from this kind of topic.
The code examples are all in Java, but you should have no problem at all understanding them because Actionscript is not really that much different at all.