I am always very polite in requesting references from providers and even thank them at the end of the email. And if/when they respond, I thank them again and offer my help in any way in the future.
Two days ago I requested a reference via eccie pm from a well-known provider who has still not gotten back with me on the reference. Her last activity on this site - 221pm yesterday.
If you do not provide references for your clients, please state as much on your site and tell your clients not to use you as a reference.