1) Start with children who don't know any better.

2) See #1.

The reason that Russia is enlisting prisoners, is because the MOD needs more people than their peace-time mandatory service contracts are able to provide, but knows that a full draft would be highly unpopular.

Roman recruitment was nothing of the sort. It was not voluntary, it was expected that everyone would participate, and there would be serious consequences for draft-dodgers, both on the level of their local communities, and on a wider social scope.

"Will service improve my personal situation? Enough to be worth it?"

Hence why recruitment correlates with unemployment.

Nations with effective recruitment policies understood this, and typically funded it with things they had in abundance (e.g. land grants, pensions, etc).