Saying no without guilt is mostly in learning the delivery. You want to convey appreciation for having been thought of, that you would say yes if it weren’t for some pesky detail in your life … So, with a slight smile, practice saying the following:
I would help you if I could, but I’m just not able to manage it right now.
That’s a wonderful idea. My schedule doesn’t allow for anything extra right now but could I call you if […]