On the condition that you do not refuse to help a person really need it, your instinctive desire to say "no" probably does not occur in a vacuum. Most likely, you clearly understand that people turn to you for help, just manipulator, which is not averse to use you for their own benefit. Such people tend to do very clearly observe their interests and to return the favor on their part can not count.
It's easy to say "no" to a stranger man, so are the people who use your education, chances are your close friends or relatives, work colleagues. If you frequently suffer from such situations when you do not have time to do their work, fulfilling requests from other, sit down and analyze which of these people are abusing it. When you realize that the person literally "sat down on the neck," it will be easy to say "no" next time.
Learn to prioritize. Perhaps the fact that you put your things for someone requests help, your business suffers, your close people who need help and attention as much as your petitioner. It is not necessary to put the interests of their own and their loved ones, is lower than the interests of a foreign man for you.
Try to refuse politely and tactfully, expressing regret and recalling the good reason that it did not seem rudeness. Give a man to understand that your worries and personal obligations are comparable to wave his needs. Put your personal well-being, inner peace and self-righteousness above grievances of the man who wants to manipulate you to achieve your goals.