Reducing Flex SWF file size and using the Flash Player cache
Reducing RSL file size
In case you are using third-party libraries you’ll never know if they heard about eclipse's build-release or -debug=false (The compiler excludes the debugging information) but what you can do is extract the SWC file and run the Optimizer on the SWF file. That will reduce those libraries size.
Improving Flex application performance using the Flash Player cache
Using RSL (Runtime Shared Libraries) can reduce your application size significantly. Using Signed RSL is much better since RSL will be cached in the flash player – that’s mean faster loading and light application file size.
About cross-domain RSLs - http://livedocs.adobe.com/flex/3/html/help.html?content=rsl_06.html
Flex compiler options - http://livedocs.adobe.com/flex/3/html/help.html?content=compilers_13.html