If your flash project includes sound, import the audio as a MP3 file instead of Wave files (convert your wave files to mp3 in your audio editing software).
This not only helps to reduce the file size of your .fla file, also reduces the time taken to publish an swf file out of it. This Projects using wave files take longer time to publish because every time it will be compressed based on your publish setting.

When handing multiple audio and video files in a project and keep on changing the audio and video also increasing your Fla file size. In my current project which is using only one wave file of 3.5 MB size. After lot of updates Fla size became 30 MB.

Reason for this is because flash file has some unwanted junk data.

Solution for this is saving the file to a different name, using file>save as menu then the junk data is cleared and file size is reduced. My Fla file became 14.45 MB after that