Here's a tip: don't import too much files.
Models, skins and icons are good, but ONLY if they actually fit your map.
A custom UI is usually not necessary and - in most cases - doesn't really fit the rest of the map.
If you use music, either remove it if it doesn't fit the map, or you can decompress it (64 kbps is reasonable for a map).
If the music is looped, perhaps you can cut it as well.
Otherwise, you can do what GhostThruster said: use Vexorian's Map Optimizer and hope the filesize will be compressed enough.