I want to say yes, but...
Someone who's important to you on some level (from a casual friend to a lover), makes a request. After thinking clearly about it, you know that, in order to take care of yourself, you need to say, "no." At the same time, you want to leave the relationship in the best possible condition. This is a way that will often work.
"A part of me would like very much to say 'yes' to you. But right now, in every way I've been able to think about it, I can't see a way where I could say 'yes' and, at the same time, take care of myself. Could you understand me?"
A fairly good friend asked me for a short-term loan, assuring me they could pay it back right away. I told them I would think about it and get back to them within an hour. I delayed responding, not so much because I needed the time to know my decision, but more to communicate that I was taking their request seriously.
Within the hour, I replied, "I much appreciate your courage to ask me for this loan. I know it would have been a choice of courage for me if I were in your shoes. A big part of me argues 'Why not? He's a good friend.' But another part just can't see a way to do it and, at the same time, feel like I was taking care of myself. I know it might be difficult to understand this. But I'm still hoping you will accept my decision."
My friend was actually quite gracious about it and said they would ask someone else.