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 explaining specifically what you appreciate is the most expressive way to say "thank you". Any phrase that you could use in more than one situation will have about the same impact as a simple "thank you". This is true for both written and spoken appreciation.
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.