At this point I'm convinced that people don't respond well to incentives that kick in over a lengthy period of time, like tax credits, monthly allowances, and childcare benefits -- as such things are already implemented in S.Korea and in other countries such as Hungary. Benefits probably need to be front-loaded, basically in the form of lump sum payments upon childbirth, though I expect this to be politically unpopular. Free housing for parents with children is another potential benefit that might be attempted, by a government brave enough.

Seems like it's time to pull out the stops, as no matter what the incentive it'll be difficult to reverse the decline as the status quo is so entrenched.