The best way to get people to do a job is to pay them enough that they choose to. If people are not choosing to join the army that means they are not getting paid enough.

This strikes me as correct in many cases, but I worry about applying the general rule to military service in particular. Soldiers who see themselves as working for pay have a lot less incentive to take on risk for the sake of their employer. And from what I remember from a few studies, offering people one kind of prominent incentive drowns out others: offer a kid a dollar for doing something he otherwise enjoyes and not only will he be less willing to do it without the dollar in the future, but he'll often do a worse job, taking no pleasure in it.