The Wordsmith's Forge

It depends on the level of gratitude/the size of what was done for me. At low levels, I typically provide sincere expression of thanks and a promise of aid in the future or a response in kind. For larger degrees of gratitude, my response is generally to do something. Most often this something is making food. I invite the person over for dinner, make them a fancy dessert or whatever else they might want (unless I already know what they like and want, I always ask). Doing something more me is an excellent way to get well fed.