On projects I've worked on previously (before I knew about Flixel!) I've often found it handy to store graphical and animation details in an external XML file (or other configurable property file).
I've found it makes it easier to maintain and makes it possible for artists to create and update alongside their sprite sheets.
I've looked around and haven't found anything like this for Flixel yet (please tell me if I'm wrong). I would like to know what you think of this idea.
Added org.flixel.FlxSpriteGraphicLoader class.
This class allows a FlxSprite to load a graphic and animations using an XML descriptor as opposed to coding the information into the class. See class for details.