its a balancing act that takes some finesse and if you are going to be a manager soon, you might look into how it is done
Im not sure if I could explain it but it usually involves listening to the request, rewording the request and repeating it back. And then explaining the situation as you see it (why you will say no) and then when they have understanding of the situation, you explain why you are saying no.
From my personal experience, this works out pretty well but there is always that one off that blows up in your face. When that happens just face it head on and as time goes on it usually gets better and requires less work when you have to say no