Bitmap graphic Images
Bitmap images such as photos take up more file size than vector graphics. Use them sparingly.
The bottom-line, always try to use a limited color palette for your banner design. Try using black and white or duotone photos whenever possible.
Try to keep your background design simple so it can be recreated in Flash as a vector shape. Radio and linear gradations are ok but patterns and sceneries will have to be imported as bitmap graphics.
Effects and Filters
Most Photoshop effects like drop shadows and glows can be reproduced in Flash using Flash filters. Always keep your layer effects active (don’t flatten them) so they can be turned off when it comes time to save slices. This results in a smaller slice K sizes. After the slices are imported in to Flash, they can be recreated without impacting the banner K size.
Limit the number of fonts and font styles you use in your banner design. Also say away from heavy stylized fonts. They tend to add more weight to your flash file.