The most important part of saying thank you in person is being sincere. The person you are thanking should understand that you mean what you say 100% and that you aren't just saying "thank you" out of obligation when you don't really mean it. Here's how to do it:  Have a sincere tone of voice. Don't say "thank you" as if it's an afterthought or as if someone else just told you to do it. Speak clearly with an even tone, and show that you really mean every word you say. Don't mumble. Use sincere words. Be specific and show that your thank you means something. Don't just say, "Thanks," but really mean it say, "Thank you so much for helping me with my homework. I never could have figured this out without your help." Be honest. Being honest is a part of being sincere, so open up and say what you really mean. Tell the person, "I don't know what I would do without you," if you really mean it. To say thank you in person, you have to show that you are truly grateful for what the person has done. You should say that the person has impacted you in some way, whether it's big or small. Your thank you should not be perfunctory, but it should show that the person's action really made a difference. Here's how to explain how grateful you are:  Be specific. Don't just say, "Thank you," but say, "Thank you for taking the time to help me pick out a prom dress. I wouldn't have been able to do it alone. If it wasn't for you, I never would have even tried on that blue dress, and now I can't imagine my prom without it." Show that you understood the person made a sacrifice. Whether the person made a big or small sacrifice to do something nice for you, you should demonstrate that you appreciate that he or she had time to make the effort. Say, "Thank you so much for letting me crash at your place last week. I know it was a really busy time for you and it wasn't easy to have a house guest, and I really appreciate that you took me in anyway." Show that you're grateful for the results of the person's help. If the person gave you an...

TL;DR in Chinese:
要真诚。 要感恩。 使用正确的肢体语言。