From this collection of Milton Friedman quotations, here is one I had not heard before:
“I do not believe that the solution to our problem is simply to elect the right people. The important thing is to establish a political climate of opinion which will make it politically profitable for the wrong people to do the right thing. Unless it is politically profitable for the wrong people to do the right thing, the right people will not do the right thing either, or if they try, they will shortly be out of office.”