[xacml] paper on applying role based access control

Somebody recommended a paper on the topic and 
 posted a pointer to the list. I looked for it, but couldn't find it.
I will be grateful if you could post the pointer again.
If you know of other (good) papers on the topic, it will be great
if you can post those pointers too.
I am hoping that these papers will suggest some "best practices"
that can be applied on our policy model/expression language to ensure
good "usability."

Regards,
-Suresh