Saying “No”, can be scary.
We all want to be helpful; in fact so many of us are so helpful to others we can't get our own work in alongside it.
The real secret to saying no so that it doesn't offend other people is to give a reason for saying no. “I'd love to help but I'm currently working on x, y and z projects and can't give it the time it deserves.” This is just one way of deflecting additional work and preserving your time.
Ask yourself: How do you think you could say “no” in polite but effective way?