Okay. For many (including yours truly), saying "no" is tough. You feel guilty, you feel selfish because someone has asked for something and you just can't let them down.
Get a grip.
"Your value as a human being does not depend on the things you do for others," advised clinical psychologist Harriet B. Braiker in her great book, The Disease to Please (2001). "Saying 'no' some of the time to some of the people will in no way diminish your value or worth in their eyes. It probably will enhance it."
Saying no establishes boundaries and keeps you from doing stuff you really don't want to do. Resist the pressure to say yes. "Learn to say no. It will be of more use to you than to be able to read Latin," advised Baptist minister Charles Haddon Spurgeon.
Make it a win-win situation by sandwiching a refusal between two positive statements: "I was really touched that you wanted me to write your Master's thesis, however I won't be able to do it. Thanks so much. I was touched that you asked me."
There. Easy, right? C'mon.
Take it from musical legend Louis Armstrong, "I don't let my mouth say nothin' my head can't stand."
Just say no.