This would depend on what you wish to show appreciation for. Search the internet for "appreciation letter" and you should find examples. These examples are classed according to what act or gift you wish to show appreciation.

Your question is quite interesting (...and challenging <3) to answer. But if you want to express appreciation by sentences, try reading sample essays or look for people who love doing creative writing. In this way, you could get ideas on how they put their feelings into words :)) Hope this is helpful :))

On what occasion do you want to use these sentences? Is it formal or informal? For friends or business? It is important otherwise we can give you the wrong advice. For example you could say to your teacher or employer: Thank you for helping me, I really appreciate your advice with this essay/assignment. Business letters of appreciation http://www.italki.com/knowledge/page/BUSINESS%20LETTERS%20OF%20COMPLIMENT.htm To a friend: Thanks for your help, you are the best! To your mother: Thanks mum, you always help me and I will never forget it.