My friend, Jane Pollak, has an interesting discussion going on in her blog about how to say 'no'. She talks about how Anne Lamott, author and speaker, creatively says 'no' to requests for endorsements and autographs.
I love Anne Lamott's work as well and had the privilege of meeting her. She is very clear on her boundaries and not apologetic about honouring them. Lack of that clarity and forgetting to take care of ourselves is what ties up in knots.
If you pay close attention to why a 'no' feels hard, you'll find an underlying assumption. We assume our 'no' hurts the other person so we want to take care of them, ease their disappointment. But when we do that we are holding the other person small and weak, assuming they can't take the truth.
The best way to say and hear no is a simple "No", clear and precise. There's no need to apologise or excuse ourselves, no need to caretake. In fact, when we dither or hold out false hope, it leaves everyone without a clear direction.
I like Paul Newman's way of handling this. If he was asked to do something he didn't want to do he replied "I don't do that". That answer is clear, indicates choice and reduces the possibility of the other party cajoling us to say yes, because pleading creates guilt which responds with a 'yes' . When that happens, we feel resentful and no one wins.
If you simply want time to think about your response then clarity is still your best bet. State your need by saying, "I need some time to think about it." Give yourself and the other person a deadline on when you'll get back to them. "Can I call you tomorrow with my answer?"
When you are clear and precise with your answers you come across as a successful person who can be counted on because when you say 'Yes' everyone, including yourself, knows that it means something.