Unreasonable Requests

Here's the deal: some requests are unreasonable. This is not the same as "out of line" or "unethical," though those requests are unreasonable and should be addressed quickly. We're talking about the requests that may prevent you for getting home in time for family dinner or put you on a red eye flight for an early morning meeting. You *could* do it, but is it necessary? If not, how do you address those requests without limiting your career?