Graphic Artist's Guild
You might also consider becoming a Graphic Artist's Guild member which give you access to their webinars for free, portfolio space, and more. But even if you choose not to, their website and blog are also a wealth of current information. This is a website worthy of frequent visits.
In this article Michelle Fifis addresses the circumstances in which you might charge by the hour. She then goes on to suggest rate ranges for specific job situations, other factors that might figure into your calculations, and offers advice on how to tell your client what your rates are. Included is a free rate sheet template.