Polite "No, thank you" professional e-mails are usually easy to compose after you've been published for a few years. In your rookie year you generally say yes to everything, mainly because you don't know any better. One or two faltering speeches at the Daughters of Nancy Drew or Sons of Heinlein club luncheons cures you of that pretty quick.
Having had enough colleagues' doors slammed in my face, I also believe in being as courteous as you can when you say no as a pro. It's hard to ask people for quotes, favors and so forth. I still pay the return postage to send back every unsolicited book or manuscript that is sent to me for quotes or votes for organization awards (on the latter, I haven't been a member of anything for years and still get them.)
I also try to explain the why behind every no, too, because I understand how a NTY can turn into Oh God She Hates My Guts and I Never Realized It in the mind of a colleague. Unless they use my explanation as a reason to start e-mail-arguing with me, and then I just have to shut it down with a "You're absolutely right, oh my, look at the time, must get back to work, see you" note.
Example: I am not now nor will I ever be interested in writing SF chick-lit. That is not because I secretly spit on chick-lit, futuristic romance, other female SF writers, or those who believe the melding of SF and chick-lit will create the next beeg genre trend. With SF, I do my own thing. I never do collaborations. Also, I know as much about feminine empowerment via fashion and footwear as I do nuclear warhead disarmament. So: Just. Not. Interested.
Here are some guidelines to politely saying No Thank You when your inbox starts overflowing with Would yous:
1. Be personal. This person thought enough of you to ask, you can think enough of them to spare them the form letter NTY.
2. Use humor whenever possible. A little laugh, even at your own expense, can ease some pain on the other side.
3. Be honest, not judgmental. It's okay to tell someone you're not into posing nude for the SFWA Writers in the Raw legal fund raiser calendar because your mom will kill you. It's not okay to suggest the requester get a body double for his/her own photo shoot -- even if the requester bears a striking resemblance to a bald warthog.
4. Say yes or no, not maybe, or probably, or possibly, etc. I had one writer do this to me when I was working on a press release list. I actually could not tell from the content of the response whether it was okay or not to add this person, and had to e-mail a second time to ask again.
5. Do not in the process of saying no ridicule the requester's charity, writer organization, friends, co-writers, or any other entity involved in the request. Exception: unless it's the Sisters of the Immaculate Love Scene, in which case, e-mail me for suggestions.
It's a given that the more popular your books become, the more often you will have to say "No, thank you." Only remember to say yes sometimes, too, especially to rookie writers looking for quotes. You never know when the next Stephen King or John Grisham may e-mail you with a nervous, "I know you must be extremely busy, but . . ."