John Jantsch has a great tip in this post titled You've Got to Sing Like You Don't Need the Money.  Here are some excerpts:

Generate more leads, more opportunities, more clients than you can possibly serve, and then raise your prices.

Here’s the theory – If I have more demands on my time than I can meet. I can look someone squarely in the eye and name my price, because I don’t “have” to get the order.  ...  Too many business owners find themselves enslaved by maniac clients that rob them of their value. Just say no can apply to marketing too.  Lastly ... nothing is more appealing than security. If a potential client sniffs even a whiff of desperation, your selling effort will move away from your terms and you won’t have the guts to name your price.

Now, I don't advocate raising prices all the time for all of your clients, but if you are serving those "maniac clients" or feel totally overwhelmed by your work load, why not take John's advice and see what happens?