When I conduct a workshop on business writing, I often ask the participants which cliché they dislike the most. The most common answer is Thanking you in advance for your anticipated co-operation.
Why? There seem to be two reasons:
First, it is normally used after a question, such as “Would you please send me the following documents?” The question implies the reader has a choice. Then the writer finishes the request with the words “thanking you in advance for … ” Now the request has become a demand, and demands tend to irritate stressed business people.
Second, people have trouble with this phrase as it implies the writer is grateful at the moment of writing but will not care when the task is actually done.
Instead of “thanking people in advance,” why not write “I would appreciate your …” ?or “I look forward to …”