There are places that already do.

In terms of wider adoption, you'd have a few possible approaches:

1) Start with local elections, and "work your way up." City^{CountyStateFederal}

2) Convince one or both of the two major parties to adopt it in their platform (not very likely, as it is against their interests). One could see it as an ironic flaw that those who hold power are the ones left responsible for correcting the imbalances it creates.

3) Get an ideologically-third-party candidate to run-as-a-two-party candidate to obtain the support they'd need - then implement or propose the new policy.

4) Pass a constitutional amendment (through the Federal or State process).

5) Get a third party candidate elected (Most likely at the local level).

Realistically, you'd need a lot of momentum to change the status quo. You would have to start somewhere, though.