Open the File-->Publish Settings menu. Under the Flash tab are two settings you can change. The first is "Compress movie;" with this checked, Flash will do its best to reduce excess data on publishing.
The second is to reduce JPEG quality. JPEGs/bitmap images are often the cause of large file sizes; reducing quality can often drop your file size exponentially. There's a JPEG quality slider that's set at 80 by default; you can use the slider to lower the quality, or enter a number in the field to the right.