The old Sun RELAX NG Converter is nowadays bundled with the (Sun) 'Multi-Schema Validator' - msv. With that term you can search in your distributions package repository.
For example Fedora (e.g. 17) includes it.
Install on Fedora
# yum install msv-rngconv msv-manual
Usage on Fedora
$ rngconv my.xsd > my.rng
Test the result:
$ xmllint --relaxng my.rng --noout some.xml
Conversion to compact syntax:
$ trang -I rng -O rnc my.rng my.rnc
(trang is also available via the main Fedora package repository)
The other way (perhaps after adding stuff to the compact syntax file):
$ trang -I rnc -O rng my.rnc my.rng
Msv seems to be quite powerful - I tested it with a fairly sized xsd file that uses inheritance, extensive patterns and includes another xsd file.