When I’m working with someone to complete a task, I want clear, direct verbiage. Tell me clearly what should be done in as few words as possible as long as the meaning comes across. ‘Please’ is always appreciated but other extraneous words and taking forever to get to the point annoys me.
So in choosing between, “Please sign this form and return it to me.”, or “Could you sign this form and return it to me?”, the first one wins. First, it shouldn’t be a question. It sounds like begging or that perhaps you aren’t sure of yourself. Second, like scaredy said, I don’t like the can/could use when I know I CAN do it. Will/would wouldn’t make it much better though. As the client, I already know the choice is mine. But you need to sound confident or I’m starting to get nervous.