We used to have a strong push every year while I was in the Marine Corps for the Combined Federal Campaign. It was strictly voluntary but 100% mandatory to contribute. Got a booklet each time that broke down who you could contribute to and the percentage that went to said organizations overhead. That was enough to make me realize that contributions when the plate was passed each Sunday morning at church was a much wiser investment.
I still gave the voluntary required minimum to the CFC, but begrudgingly so.