There is an awesome article posted on Freelance.com called How to Start Charging Your Dream Rates (and Get Away With It)
Skellie writes about the three factors that will determine whether your clients accept your “dream rates”:
1. They need to believe you’re worth it
2. They must be able to afford it
3. They need to believe that you wouldn’t do it any cheaper

I really liked some of the phrases he used to illustrate point number three. He writes:

If the way you communicate portrays a lack of confidence in your ability to charge the rates you want, clients will pick up on it. Phrases like: “Let me know if these rates suit you”, “But I’m open to negotiation,” and “Let me know if this is more than you can afford” all say one thing and one thing only to clients: “I hope you’ll pay me what I’ve asked, but I’ll easily work for less“.

Are you guilty of using any of those phrases? Instead, he recommends using phrases like “My standard fee for all clients is…” or “The minimum rate I do this kind of work for is…” I agree that this will discourage people from trying to negotiate.
Just remember, not everyone should be able to afford you. If everyone can afford you, you are not charging enough money. I would add to his list a fourth point. YOU must believe you are worth what you charge. So much of rate-setting is about convincing ourselves that we are worth charging our dream rate.