Once you think that you do more than they pay you, you can discuss the issue with your boss directly. Tell him that you spend more time on the job, sometimes you even work over time. Besides this, you think about the job, and find the ways how to complete the work at home. You need continue to do this in the future. You can say that I hope you can increase my pay based on my extra work. It's hard to open your mouth on this issue, but once you do that, the boss will give you something if they know you are important. Good luck.