I've found the resources the folks at Basho (makers of Riak) have put together on their wiki and blog quite useful. The following would be a good starting point:

http://blog.basho.com/2010/03/19/schema-design-in-riak---int...

http://blog.basho.com/2010/03/25/schema-design-in-riak---rel...

http://wiki.basho.com/

http://blog.basho.com/