My rule is choosing one pro bono project per year. But I treat them like a “real” client with the same priority as if I was getting paid. I try and do the best for them I can. I usually include “teaching and coaching.” And yes, a contract or agreement is necessary to establish good boundaries.
I do not put design credits on my work. Advertising is not my motive. I simply want to do something good for someone. Who’s the most deserving is a personal choice. If the work is creative, aesthetic, and succeeds at accomplishing goals, word gets around.
So far these have been some of the most difficult people to work with. You really have to spend time educating them about why design is important and how it can affect people. They frequently expect rush work as if they were overpaying you (or creative ideas just fall out of your head on-demand.) So it requires some fortitude.
In some circumstance, I had to quit. They were becoming too parasitic or demanding. That’s why I think one project a year is plenty ulcer for most designers.
We’ve been given gifts and need to payback” something to the world. Pro bono is always needed. The best, of course, is when a pro bono client says, “You’re the expert. You do it your way.” Nice.
But generally, pro bono work requires some suffering. It’s a sacrifice of love for people and for design.