I once found this instruction for how to express gratitude that I find very useful:
Never take more than you need.
Use all that you take.
Thank the person for the gifted thing or favor, and tell them how you have used or will use the gifted thing or favor.
Personally, what makes me feel good about giving people things or doing favors to them is to know that they plan to use them well, or even better, that they already have used them well.
A thank-you is appreciated, but it's much better to know that the resources and favors one has given others or done for others have been used well.