A common technique is to quietly raise the rate 10% on a per project basis. This is easier to do if you bill at a per project quote flat rate vs T&M (Time and Materials). If you have a full time job, it is less stressful to do since your dinner doesn’t depend on it. As a student I think $20 is reasonable (you are a student after all), but now that you have graduated, I would double the rate and go from there. In the end, you may have some legacy clients who lag behind in the rates, but stay firm with new clients.