I had to do some simple XML schema and needed different formats : DTD, maybe xsd and I wanted to keep writing easy, so I choose to use Relax NG Compact.
It is easy to find ressources on how to write such RNC Schema, (if you are a bit used to DTD it is even easier.)
Two main ressources for RELAX NG : www.thaiopensource.com (you will find Trang.jar : to convert between schema) and relaxng.org.
The Sun RELAX NG Converter doesn’t support the compact format and I couldn’t convert from DTD to RNG.
So, Trang did the trick, (sometimes, their site is down, try again) !