I tend to rotate whole armies through active service. Not the most efficient, since it means I'm paying upkeep for an army that's not fighting for 3-4 turns at a time, but it does mean I've always got a reserve stack between the front line and my production cities in case things go pear shaped.

At the moment in my english campaign (2tpy, turn 120ish) the king is in eastern france with an almost full stack, while my other (replenished) army is waiting for ships at dover, having fought a few battles in france, been relieved by the king and taken a trip back to blighty to be spread between London and Nottingham to stock up on recruits/the latest armour.