Although Romney had a good chance to win the election, he was also pretty gaffe-prone, and overcoming that would be difficult for him. A lot of his worst statements (being "severely conservative", having "binders full of women", and the whole 47% thing) are difficult, but admittedly not impossible, to avoid. That having been said, he also strikes me as the most likely Republican to pull off a victory over President Obama. First and foremost, I'd say he needs a different running mate. Paul Ryan was a safe choice, but with that, an uninspiring one. Going up against President Obama, Romney needed to make Americans believe that he had a vision for change and "hope" of a different variety, and making a trust-fund baby like Ryan his running mate didn't help. My suggestion would be to go with Marco Rubio. Young, Hispanic, the son of immigrants and staunchly pro-life. This, in and of itself, could help to hand Florida to the Republican ticket, since it came down to less than a 1% difference IOTL. Florida alone wouldn't be enough to win him the election, but it's a start. Secondly, assuming that his gaffes are unavoidable, Romney at least needs a better ground game. He was very convinced that he'd win, so to the point of not even writing a concession speech, and as a result he didn't work too hard in the states that he needed (and very possibly could've won), including Ohio and Virginia. So if we take Florida, Ohio, and Virginia, and assume that he hit all of them enough to eek out wins there, that brings him to 266; close, but not quite enough. But if we assume that Romney has an improved ground game in Ohio, an absolutely pivotal state for Republicans, it's not wild to imagine that he'll do the same in Pennsylvania. If he can win there, too, that hands it to him with 286 electoral votes. It's tight, but it's a win nonetheless.
Not terribly specific, but barring changes to Romney's personality or ASB'ing away Hurricane Sandy, this seems like the best way for him to go.