A Thank you note to a teacher - opening sentence?

I want to write a thank you note for a teacher, and I know what I want to write, but I just cannot get the right words together for my opening sentence. I have had this teacher for four years, and his teaching and class mean so much to me, that I'm scared I might sound ridiculous in my letter. Any help?

Let your teacher know how much you appreciate his help and then you can elaborate on what you intended to say to him.
---- Hope this helps...

Write down several sentences as you can, then choose one of them and edit it again if you need. --- I think this helps

You won't sound ridiculous, friend! Just post your paragraphs here, and we'll correct any errors. Writing the first sentence is one of the most satisfying pleasures when writing any essay or letter -- especially something like this that really comes from the heart.

But here is the strategy: Write a sentence about each point you want to make, and build the paragraphs of this note... then, while you work, one great sentence will come to mind, and it is that sentence you should use to begin the thing.