Recently I read an article over the way that one should use should ask for a raise.
The biggest problem is that we do not know our worth.
It is an essential element needed for your negotiation.
A good way to check your worth is via Internet tools which show statistical information from people who entered their salary, experience, sector in which they are working on and education level.
Another way, the one which I suggest to you since I think it keeps people sharp, is to quietly look for open jobs, similar to yours and have interviews. It shall give you a better idea of what you worth, it will increase your negotiation skills, it will expand your network and it will create 'open doors' in case things go wrong during your raise negotiations..
Also, you must be able to present the situation in case you would not be there.. In other words, what is the added value of having you at the position you are now and how do you make profit or add value to the department/company? Why should you get a raise? Why you and not somebody else?