Two things that I believe would help:
1. Don’t include MIDI music - This prevents people who download the “ripped” versions of the games from having any music at all and this makes it much less worthwhile for them. Think about this: How many pirates would have enjoyed Season of the Sakura as much as they did if MIDI music had not been included? Same thing applies to Tokimeki Check-In, someone could download it and get all of the music in about 1MB or less of downloading, even if it is lower quality than the CD, it certainly makes the game a lot more enjoyable than silence.
2. Make the games require the CD’s to run. This way, it makes it more difficult to get games from the net as people need to create CD cracks. I know this won’t prevent all piracy by any means, but it will certainly make it more difficult. As it stands, I own Snow Drop and Tokimeki Check-In and it would be very easy for me to install these games on a friends computer and they could run them with no loss in quality (Snow Drop) or little loss in quality (Tokimeki Check-In) without any access to the original CD’s at all.
3. Take requiring the CD one stage further, make the games executable check that one or more very specific large files exist (Such as the voice or music file packs) before starting the game, if the files don’t exist or the file size is incorrect, the game doesn’t run.
My point really is that many people are happy to rip-off games that don’t have voice and only have MIDI music, but far less are willing if it means downloading all of the voice and music files or trading / buying CD’s by post with someone like z3n (I already fell for that one myself. ).