Writing "thank you" note to professor who will write more LOR's? I have been working with a professor for research for over a year and will continue to for at least another year.

The professor has written several letters of recommendation for me in the past, and has agreed to submit more in the coming years. 

Should I write a "thank you" note for the past letters, even though I still work with this professor regularly and will likely ask for more letters in the future?
If you are worried that sending such a letter would be something of a bribe for the future letters, you can rest easy. No one is likely to think that. But there may be a lot of reasons to thank professors, including what they contribute to your education. 

But in the case that someone writes you a letter and you get an award or offer partly as a result, then you should definitely thank them.