Preferential voting means no dead votes and influences the mandates of the big parties in Australia.

Say for US it meant you directly preferential vote for your candidate.

You could vote
Independent 1
Romney 2
Cain 3
Obama 4

Votes are totaled up. If no one has a majority the lowest scoring candidate gets removed.

Your favourite indie fails. Your preferential vote goes to #2.

Say at this point Romney wins. A lot of votes he gained were from the indie. Here in Aus the candidates adapt and will generally bring in a few of the ideas from candidates they gained votes from... Mainly to shore up their votes in the next election cycle.

Probably have a bigger impact on congress as it would mean voters would be more Willing to vote for long shots they agree with, and have the insurance that their vote will still count against a disliked candidate.