In my opinion, nothing. Microsoft has a grip on the US and with the release of Titanfall that grip will only strengthen. We Americans do love our shooters.
Sony hit hard, but is still being outsold in the states. Now, if the real issue the Ps4 is getting handed to by the Xbone is supply, then this thread is irrelevent.
But, if the Xbone actually has a larger demand than the Ps4 in the US, then Sony needs to act quick if it doesn't want to end up last the Ps3. Once the dust settles and gamers choice what platform their friends are playing on, it's hard to overcome that userbase.
So, what can Sony do to take the US with Ps4? Is it games? Is it one game, or many? Continue great P Plus support?
All Sony has to do is to continue what they've been doing, release the planned games, and ease out the supply. This is enough of a reason for Sony to beat MS in US. There are 4 major indicators for this:
a) There is obviously more demand for PS4 in US, regardless of which metric or source you look at. Demand does not equal sales unless supply and logistics are appropriate but Sony will eventually get it right. MS, admittedly did a better job for relative supply in US, and got rewarded handsomely in return although this will not turn to a long term impact
b) PS4 is heavily under-supplied, and this will change. In the worst case scenario, where the same demand will hit both PS4 and XB1 supplies.
c) PS4 is cheaper, and even with equal levels of demand, it will sell roughly 20% more than XB1 (in US).
d) UK is a good indicator of things to come. UK had relatively abundant supply of PS4, and easily sold 20% more than XB1, and still sold out meeting much higher demand. US will repeat a somewhat more moderate version of that.
Either way PS4, will be ahead of XB1 in US, but neither will dominate. The overall XB1 sales ratio will be between 35%-45% against PS4, which can actually prove better (mean more sales) for MS than the last generation, given the lack of an overshadowing WiiU.