There are a number of conventional wisdom's when it comes to choosing a VP. In 2008 McCain chose Sara Palin. At the time McCain was correctly percieved as weak with the GOP base. He also trailed with women. The idea behind the pick was to solidify those two demographics for McCain.
The strategy was correct in 2008, the candidate they chose was the problem. Palin proved to be a divisive character. Many moderate Republicans and many more Independents were turned off by her lack of experience and apparent lack of depth of knowledge on National and International issues. 2012 is no different. Romney needs a splash with the veep choice. Romney polls very well with white males and wealthy suburbanites (Obama's core demographic in 2008). Romney does have issues with latinos and women however. His poll numbers with Latinos in particular have been dreadful. Recent polling show as much as a 73-27 lead for Obama among Hispanic voters. His hard line immigration stance in the primaries didn't help there. Romney has also lagged a bit (between 3-7 points) with women in most polling.
Politically speaking the ideal VP candidate would appeal to one or both of those demographics. The difference today from 2008 however is that this time they must find a "qualified to be president" version of Sara Palin. VP's are often chosen to balance out a ticket. Obama had no experience when he ran and also was percieved to be weak with blue collar voters (his strength was urban voters and white upper income suburbanites) so he took an old establishment Democrat white guy that supposedly appeals to blue collar voters and had Foreign Policy experience. Romney's percieved vulnerability is with latinos and women. His choice needs to fire up one of those groups.
Notice I didn't say he needs to fire up the base. Thats because the base is fired up already to get Obama out. There is no need to further placate them with a red meat pick like Demint or Ryan or someone of "Tea Party" ilk. It wouldnt hurt if the VP pick that appeals to Women or Latinos also fires up the TP base but it isn't a must.
Politically speaking the top choice for VP should be New Mexico Governor Susanna Martinez. Governor Martinez story is compelling. She had a middle class upbringing. Her father was a boxer and eventually became the Sheriff of his town. Martinez was a very successful prosecutor before becoming Governor. She was known as a top child advocate as prosecutor. She is also very strongly Pro-Life and Pro-Gun. She speaks with the comfort and ease of a former prosecutor. She also cares for her disabled sister. In my opinion Martinez hits every nail on the head as far as the checklist goes for VP. Romney would be wise to choose her.
With thats said, Romney is considered "weak" with women and Hispanics because of Liberal Feminism and Illegal Immigration Supporting Hisapnic groups, not because he's actually weak on policy for either group as a whole. So adding an hispanic woman will not help him with those he's weak with, as those folsk will never vote for a Pro-Life or Anti-Illegal Immigrant choice anyway.
I do think the VP choice can make a difference. It isn't the end all but they could excite a particular demographic. i have been hearing some buzz around David Petrayus (sp?) as a potential running mate as well. Brings Foreign Policy cred.