I think the easiest way is -
1. Be Empire culture (they have far more towns and villages than anyone and are centrally located)
2. Have Combat Tips perk to boost level of recruited troops sharing your culture (25 Leadership)
3. Get lots of rep with Empire notables (do quests, leave high charm heroes in taverns, at 100+ smithing do orders, etc.)
All this applies for other culture choices, but it's way easier to manage as Empire w/Empire troops.
It also always helps to have a high level party, use good tactics, have high medicine or a good medic - these all reduce the amount of troops you lose in battle.