The ways to thank someone are endless, but "I cannot thank you enough," and "I am truly grateful" are good starting points. To be thanked, be it at work or at home, is to be appreciated, and it is equally important to let others know that they, too, are appreciated.
Other sayings that express one's gratitude are:
"The smallest act of kindness is worth more than the grandest intention." Oscar Wilde
"No duty is more urgent than that of returning thanks." James Allen
"One of the deep secrets of life is that all that is really worth the doing is what we do for others." Lewis Carroll
"My world is brighter and more beautiful because of you. Thank you." Unknown
"Thank you for coming into my life and giving me joy, thank you for loving me and receiving my love in return. Thank you for the memories I will cherish forever." Nicholas Sparks