As I see it, you have probably two ways.
You don't *really* have to publish a lot, but that certainly will help you a ton if you go the NPP route. NPP is only really good if the center you are applying to already knows you. Or, at the very least, they know your work. Basically, when you apply to a NPP, you apply directly to a group at a specific NASA center. If they know you, you have a much better shot of them saying they want you. Go to conferences, especially the smaller ones that are just focused on one topic. That's where you'll meet the NASA people that work in your field of interest. If you can show them you do good work that they like, you can improve your chances at NPP drastically.
The other way to do this is to apply to a graduate school who has an established connection with a NASA center already. That way you make a crapton of NASA connections while a graduate student, and are already familiar with NASA center protocols.