Ask yourself if this is work you _want_ to do. What purpose does it serve - not just those you're teaching, but yourself? Can this be something you learn from too? Can it help open doors?

One thing to remember: people value work that they pay for. Something that is given freely has no value. And that's not something that can be changed.

If you do say no, say no as politely as possible. Make the no an opportunity, rather than a closing of a door.