My creative writing teacher in high school said "the specific is terrific."

Observe and think when you say "Thank you for A, it really helped me because B", where A and B should be detailed.

Frequently people don't have a clear picture of how what they did is important to the business, how helpful the thing they did for you is, etc. So make sure they get that.

Another thing is to give small gifts. I live on a farm where a friend has quite a few beehives. We have honey coming out of our ears (some of the best tasting honey there is) and we give jars away all the time.