I have the bulk of AS3 memorized. I just write the code, similar to writing this post.
I keep the book "Essential Actionscript 3.0" by Colin Moock, next to my computer for reference.
IMO, the best way to learn the language is to start with Events and Displaylist tutorials.
Also, start writing your code in external AS classes (OOP). The learning curve is slightly steeper, since you have to learn class syntax, but ultimately it's the right way to do it.
I'm currently exploring the GAIA framework. I really wish I had known about this when I first started with AS3. Amazing results and very simple to work with.
This framework is an excellent example of "best practices" for beginners to learn from.