I use flex states a lot. I think they're a great way to handle navigation within your application. However, I have recently discovered that any mxml component added with AddChild will not and can not be removed from memory. This isn't a problem with small applications, but with large ones, this is pretty awful. It's not technically a memory "leak" because going back and forth between two states won't pile on the memory, but every state you navigate to will never be released.

The garbage collection in Flash is all done behind the scenes, and it takes some tricks up the sleeve in order to ensure things get garbage collected, and also to check whether things are garbage collected.

This article has simple techniques as well as advanced. Read or skim what best suits you.

The first thing I want to write about is how to decide when and where to optimize something. I've observed that some people care absolutely nothing about optimization as long as the program works, and some people waste altogether way too much time optimizing things that make little to no impact.