Apologizing for not being able to say yes may be ideal so that it will be more respectful. At times, apologizing may help the person convince you to say yes eventually.
There are some things that you can say “NO” to but there may be other things that you are willing to do so that is the time when you would answer yes. This means that you can help the person without having to do something that you are not comfortable doing. This can be another respectful way of saying no, right?