Pay review is the time you find out just how much worth you have as an employee. Back up your requests with evidence that you deserve a rise, how well you have performed against targets, exceeded goals etc etc. If they refuse, hand in your notice. It's at that point that you find out just how much you are worth!
if you don't have annual performance reviews/salary increases in your firm, ask your boss for a discussion. Sit down and ask him/her if he has been satisfied with your work over the past x months. List any extra tasks you have taken on and how you think you've performed them and any extra skills you have acquired. Tell him that you think you deserve more money. If you have the same skills and experience of others in the company who are earning more than you, give examples to show that you might be being paid less than the market rate.