Implement campaign spending limits, regulate or ban PAC's, and commit to an ongoing effort to stomp whatever new methods big-money comes up with to influence politics.

We do most of this in Canada and our leaders seem to be less influenced by big money. (Nevermind that we recently elected a billionaire PM...) The vast expense of running a U.S. style election campaign virtually guarantees that U.S. politicians are all bought and paid for.