That is a deep question :-) I answer a part of it in chapter 1 of the book (freely downloadable from
http://manning.com/laddad2).
Fundamentally, with AOP you need to follow the Single Responsibility Principle (SRP) to its logical end. Classes only worry about their main responsibility, since aspect take care of crosscutting concerns that would have been otherwise fused with classes. With this style of thinking, aspects appear naturally in the design and implementation process.
-Ramnivas
