First thing, have a productive economy whose benefits are widely shared.
Seriously, if prospective parents look at the future and see no happy ending ahead, but just a life where every few years, you’re thrown out of a job and have to spend what savings you have just to keep afloat until you find decent employment again, no, you’re not going to be eager to have kids.
That’s a lot of people’s experience, these days. And the economies of countries like Spain make the U.S. look like paradise by comparison.
Immigration is obviously another way to solve the birth dearth, to steal Ben Wattenberg’s phrase. (Good Lord, we’ve been talking about this for half my life! That book came out in 1987.) But when there already don’t seem to be enough jobs to go around, you’re not going to want to let in more people to fight you for those jobs.
Second, make being a parent more affordable, in terms of both money and time. Paid maternity leave at one end, affordable college at the other. (Starting in the 1980s, conservatives argued that the taxpayers ought to pay a bit less for college, and the students themselves - the direct beneficiaries, ought to pay a bit more. Made sense then, maybe, but now we’ve gone way too far in that direction. And the benefits of a degree are a lot less of a sure thing at the individual level than they used to be.) And in between, in an era where many people can’t get by on one income, subsidized day care, subsidized before/after care through the pre-teen years, and subsidized summer programs for the kids that are compatible with the parents working full time.
And of course, universal affordable health care, because giving birth places great stress on one’s body as well as on one’s everyday life, and while most women recover from labor without long-term consequences, those that don’t recover quite so well need to know that they’re going to get the medical attention they need.
That’s not a complete list, but I think it hits the high points.