Ask for a raise
There is an old saying, but it is a good one: “You get what you settle for”
Sometimes making more money is simply a matter of asking for it. Now timing and common sense are very important here. You don’t want to ask for a raise right after returning from a long vacation, unless you’re sure they were struggling without you. Don’t go storming into your manager’s office making demands if he or she is obviously in a bad mood. In fact, you should try to look as objectively as possible at the value you are bringing. This is hard, you have to put your ego in check. Write down 5 ways in which you bring value to the business that show you merit a raise — then write down 5 reasons why the company can’t afford it. How do they compare? You have to be prepared to make your case for the increase and be prepared for a negative answer. Have your contingency plan in mind.
Remember, all too often, if you don’t ask for it you won’t get it. It is not uncommon for a boss to not realize that you desire a raise, especially if you are doing a good job, he or she might just think you are content. Hey I said you have to have common sense - don’t necessarily expect it from your boss! 
More on the subject:
SoYouWanna: Ask for a raise?
eHow: How to Ask for a Raise
The Career Adviser: How to Ask for a Raise