'No' is a complete sentence.
You can't soften it in a number of ways:
- I don't want to do that (right now)
- I'm not able to do that (right now)
- I'm not the best person for that, etc.
I often don't explain.
Explanations can look like the opening move in a negotiation.
If it's not a negotiation, don't explain.
If you really have to explain, keep it to one sentence.
Alternatively, something neutral like, 'I'm not available', 'Personal reasons', or 'Family reasons' is usually sufficient.
People who respect boundaries don't want to know and don't need to know.
People who believe they have to have an explanation are usually not satisfied with the explanation provided, so it's useless to go down that track.
The other thing about explanations is that they're almost always insulting: if I turn down (b) because of (a), and explain why, I'm telling the offeror of (b) why (a) is more important to me than they are or (b) is. No one likes to be told why they're being passed over for a better offer.
Where there is a genuine emergency, and that's why I'm saying no, I try not to cite that any more. Why? Because most people think you're lying anyway, and it doesn't mitigate the risk of being considered unreliable. There are people who are considered unreliable because 'they're always having emergencies'. You don't want to be placed in that category.
It's also helpful to be unequivocal. 'I'm not available' is better than 'It's pretty likely I won't be available, so, the way things look now, I think on balance it's probably a no'.
The other tool I use a lot is this:
Silence is also a complete sentence, especially in written communications. It is often capable of sending the message you want to send, instantly and effectively.
So, rather than even saying 'no', just ignore it. They'll get over it. You're not that important. 😉