This is somthing I learnt from trial and error. Saying No at the workplace is one of the most difficult things I ever faced. The below excuses are some of the best ones I've used in various situations.
1. "I CAN'T RIGHT NOW, BUT I CAN DO IT LATER"
2. "I'M REALLY NOT THE RIGHT PERSON FOR THIS"
3. "I JUST DON'T HAVE ANY ROOM IN MY CALENDAR RIGHT NOW"
4. "I CAN'T, BUT LET ME GIVE YOU THE NAME OF SOMEONE WHO CAN"
5. "I HAVE ANOTHER COMMITMENT"
6. "I'M IN THE MIDDLE OF SEVERAL PROJECTS AND CAN'T SPARE THE TIME"
7. "I'VE HAD A FEW THINGS COME UP AND I NEED TO DEAL WITH THOSE FIRST"
8. "I WOULD RATHER DECLINE THAN END UP MESSING THIS"
12. "I CAN'T, BUT I'M HAPPY TO HELP OUT WITH ANOTHER TASK"
13. "I'VE LEARNED IN THE PAST THAT THIS REALLY ISN'T MY STRONG SUIT"
15. "I DON'T HAVE ANY EXPERIENCE WITH THAT, SO I CAN'T HELP YOU"
16. "I'M NOT COMFORTABLE WITH THAT"
17. "I HATE TO SPLIT MY ATTENTION AMONG TOO MANY PROJECTS"
a simple, straight, direct "NO"