If you want to say it casually - Thanks for your help. If you want to express that the person has been very kind - 1) Thanks for your kind help. (When someone helps you by answering your questions or pointing you in the correct direction.) 2) Thanks, you've been very kind. (If someone help you and the act of helping you took some time, eg. walk you to the place you are looking for) 3) Thanks, that's very kind of you. (Some one picked up something you've dropped or gave their seat up for you) Hope this helps.