My Recommendations for Romney

It looks pretty certain now that you've got the Republican nomination, unless it turns out you're a secret polygamist or something equally absurd.
Therefore your next task is to get a running mate.
I'm going to caution you against the urge to grab a Hispanic, Woman, or Black.  None of these options will likely play well for you.  Rubio would probably play the least badly of those three if you just feel totally compelled.
Instead, go 'boring'---IE a straight white man with children, with a small twist.  Get Rand (not Ron) Paul.  Rand will give you most of the positives that Ron would, with much much less baggage.  You'll likely inherit a fair bit of support from the Paulites in general by doing this, especially if you sweeten the deal.  Here's how you sweeten the deal:

Seriously tone down the rhetoric on 'invade the world'.
Talk up substantially the notion that the 50 states are laboratories of democracy and they need to be allowed as free a hand as possible, but that some of the things cooked up in a given state are NOT suitable for national export (read, Romneycare).  You've said previously that you'll immediately issue waivers to Obamacare to all 50 states if elected.  You need to repeat this message.  Because of the Massachusetts health care example, you need to be extremely clear as to your intentions.  Also, this would be an excellent opportunity to 'dog whistle' to the marijuana legalisation/decriminalisation crowd that you have no intention of using the federal government to override their initiatives in their states should they manage to get them passed.  Here's a line you might sharpen up:

My administration has no intention of getting involved in what should be a state or local criminal or civil matter, especially when such is the manifest will of the voters of that state.  I don't believe that the purpose of the federal government is to bully the states.

Finally, I hope you've got a thick skin.  You're going to need it.  The kitchen sink is going to be thrown at you by the MSM.

Update:
http://legalinsurrection.com/2012/04/cnn-liberal-commentator-ann-romney-never-worked-a-day-in-her-life/
They've started attacking your wife already.  This is actually a good thing for you.  Stay at home moms actually aren't all that rare and they'll remember this insult LONG after everyone else has forgotten.  Women have a long memory for insults and slights, something I'm sure you're aware of having been married for a long time.