Rush Limbaugh said on the radio last week that if everybody can sway just one undecided voter, Mitt Romney would handily win the election. Now, I don't believe Romney will win, unfortunately. But I am an eternal pessimist. For those readers more optimistic than I, this is how you persuade undecided voters to join Team Romney.
Pick an undecided voter you know (or one you don't know---online, say), and ask him calmly, "What do you like about Obama and what do you like about Romney?" Make note of his answers, but before responding, also ask, "What do you dislike about Obama and what do you dislike about Romney?"
Keeping in mind the reasons he likes both candidates and the worries he has about both candidates, talk with him about why he should strongly consider voting for Romney. Chances are that the reasons he likes Obama and dislikes Romney are misconceptions about the two candidates. This is usually the case, in fact, judging by my experiences with undecided voters. So correct those misconceptions.
The most important thing is not to use emotionally charged language. Do not, for the love of God, call Obama a socialist, a Marxist, or even a radical. Do not. Obama may be all three of those things, but calling him one does not convert anybody and, in fact, tends to turn people against you, not against Obama. Calling him a socialist (Marxist, radical, what have you) is also much too vague. When decrying socialism, one doesn't simply say, "Socialism is bad because it's socialism." It must be explained why socialism is bad. Likewise, we must explain what specifically makes Obama's policies bad. "Because they are socialistic policies" is not a convincing argument. It isn't even an argument.
Another good thing not to do is not to overload the undecided with too much information in one sitting. If the person you are trying to convert is someone you will be communicating with again in the near future, save some information for that later date. If the person is someone you will never see or speak to again, pick only one or two critical reasons not to vote for Obama, but make sure also to pick one or two critical reasons to vote for Romney. People want to vote for someone rather than just against someone.
Do not pressure the undecided voter. Allow everything to sink into him. He will not make up his mind that day. It could be several weeks before he decides. Do not insist that he have an answer to "Who are you voting for?" immediately after you've spoken with him, because he won't. And that's okay. Be patient.
Good luck. Hopefully you won't need it and my new-found pessimism for this election is not in fact a case of realism.