Salary should be agreed upon when signing your employment contract at the onset. However, dependant on your duties and if your duties change over time, there should be a contingency clause within your contract that denotes that your rate of pay will be incrementally increased when & if your role changes. This aside..It doesn't hurt to have a sit down with your employer to outline the skills & experience you bring to the position, including extra duties you have taken on in the course of your employment that would justify a raise in hourly rate.

All in all...be well prepared when negotiating pay, as you will need to be able to sell your employer on what you believe you are worth. All the best of luck.