Is there anything else other than 'thank you' to show gratitude in a polite, formal way to my professor at college?
My experience is that saying specifically what you appreciated is the most expressive way to say thank you. Any phrase you could use in more than one situation will have about the same impact as a simple thank you. In my experience, this is true for both written and spoken appreciation.
For example, "Thank you for helping me outside of your normal office hours to accommodate my work schedule. Your generosity with your time really helped me." is much more meaningful than "Thank you for all that you've done for me."