Asking for a raise can be anxiety provoking; therefore, many people put it off because they are afraid. A very common error is to hope the problem will be magically solved and your boss will automatically remedy the situation for you. The second most common mistake is allowing the built up resentment of feeling underpaid to grow to a point where you begin making aggressive demands.
Generally, neither approach is successful. If you are hoping for a pay increase, here are some things to take into consideration before having that conversation with your boss.