Modules are probably the single biggest opportunity for most applications.
Embedded assets really need to justify their inclusion.
You can separate the flex framework from your main .swf, but whether this is a nett benefit needs looking into on a case-by-case basis, IMO.
Do not use extra layout containers - like HBox and VBox - when you could just use Canvas. Do not use Canvas when you could use UIComponent, and so on.
Develop a nice preloader, as that buys you a couple of extra seconds.