Decide whether you want to do what you ask. When you received an offer, you need to understand for yourself whether you really want to do it. To respond to the proposal only when you clearly define you need it or not. Tell yourself: "No, I don't need it!".
Say "no" to his companion. Don't be afraid to offend people. If you do it correctly, resentment or overt anger will follow. Explain your failure. Give reasons why you are unable or unwilling to fulfill the request. In conversation, often use the pronoun "I". Speak clearly, not confused. Do not make excuses, and argue it!
What are the reasons of failure. The reason may be both real and fictional. However, remember that it must be clear to the interlocutor. It needs to agree with you and accept your failure. Don't be rude and don't be cutting. Speak calmly, keep your eyes on the nose of the interlocutor. Shifty eyes and uncertainty in her voice can give the other person understand that you feel uncomfortable and he will push you.
Refuse, making the compliment. Denying, tell the interviewee something nice. For example, you can say, "Great idea, but...". People need to understand that you want to fulfill his request and, if not for the circumstances, you certainly would have fulfilled it.
Repeat your refusal. Psychologists say that a person needs to hear a refusal three times before he realizes that getting consent is impossible. Be patient. All entreaties answer "no". Be calm and control yourself.
Practice with your friends. Ask a friend to pester you with any request. Deny him this. Ask him to point out your flaws and errors failure: the shifty eyes, hesitant voice, the softness. Over time, the waivers will be given to you much easier.
Remember: when you deny a person you don't deliberately hurt him, and do what you need.