Is the majority of the game audio or graphic files? You can find this out by looking into your game folder.
Highlight Audio, and right click properties. If the majority of your file size is here you can download audacity to help convert mp3 files into .ogg. Make sure you select File>Export>Select .ogg format, and under properties reduce quality to "0".
For about 30 songs it took me an hour or so to take a 212 meg demo down to 86 megs without RTP. Use only songs for the demo, not for material that people won't be playing.
Highlight graphics and right click properties. If the majority of file size is here it may be time to trim the tile sets, characters, and non rtp materials your using to a more "bare bones" state. Like with the audio, only include material for the demo people will actually see.
If you are worried about trimming too much, save the game as a separate project or copy it's folder and name it "Demo".