How and when to raise your rates

by Corinne McKay
http://thoughtsontranslation.com/2014/02/10/raising-your-translation-rates-how-and-when/

Short answer: with new clients, and when you’re already really busy.
...
If 100% of potential clients accept your rates without negotiating, you could be charging more. 

And here's a neat PROZ page that shows members' average reported rates for all language pairs.
http://search.proz.com/?sp=pfe/rates

"Interpreting: Getting it Right" (Guide for interpreting buyers, by the ATA)

For non-linguists. A straightforward brochure that explains the where, why, and how of professional interpreting services. It's a quick read that offers practical, hands-on information for language services consumers, perfect for client education.

http://www.atanet.org/docs/Getting_it_right_int.pdf

Pricing Strategies for Translators and Interpreters

By Judy Jenner
http://www.atanet.org/chronicle/feature_article_june2012.php

(Proz.com award-winning article)