Well, actually, don't just say "No!" Say it with style...
Many of my clients have time management issues. Sometimes it's from difficulty with managing the concept of time, but often it's because they have filled their schedules by saying "yes" too many times to too many people.
ADDitude Magazine (a magazine devoted to adults and families living with AD/HD) recently ran a sidebar called "12 Clever Ways to Decline," written by Ramona Creel of Onlineorganizing.com. I'll add my cleverness at the bottom.
12 Clever Ways to Decline
1. I'm in the middle of several projects.
2. I'm not comfortable with that.
3. I'm not taking on any new responsibilities.
4. I'm not the most qualified person for the job.
5. I do not enjoy that kind of work.
6. I do not have any more room in my calendar.
7. I hate to split my attention among projects.
8. I know you will do a wonderful job yourself.
9. I need to leave some free time for myself.
10. I would rather help out with another task.
11. I have no experience with that.
12. I have another commitment.
Personally, I'm a little leery of #10 and #11. Only use #10 if you really want another task, and you're not trying to get out of doing something. And with #11, the other person can too easily come back with "So this is how you would get some experience." That's been used on me! For me, #11 is only good for things like flying an airplane, surgery of any kind, higher math and giving hair cuts.
Here are my additions:
13. Let me check my calendar and get back with you.
This gives you time to decide which "no" you are going to use. You also get to confirm for yourself that your calendar is full. Make sure you have filled your calendar. You know you can fill your calendar with things like "Bubble bath: 6 pm," "Nap: 2-4 pm," "Wash the dog: 11-12 pm," "Play computer solitaire: 7 am -3 pm." I'm serious, be sure to schedule all the fun things you like to do. (Well maybe computer solitaire for 8 hours is a bit extreme.) But it helps you avoid the next excuse...
14. Oh, the last time I did that they had to call the fire department.
Now this is a very specific excuse. Feel free to substitute a more appropriate agency–paramedics, police, park rangers, health inspector, social services, the IRS, Coast Guard, National Guard, Marines etc. Yes, this one does border on the "little white lie." (I hope.) But it gets the other party to say no, not you!
So, if you think you would have trouble using any of these, I suggest practicing. Take the ones you like, put them in your own words, stand in front of a mirror and repeat them until they roll off your tongue. Call it a fire drill if you like. When the time comes to use one, it will be right there for you, sound natural (do give a short pause first and look like you are mentally checking your calendar) and get the job done. The job is to avoid a job you don't have time for or just don't want to do.
It's been nice "No-ing you!"