To answer the original question though, since you aren't asking about a particular rush it's impossible to suggest a particular counter, but there are a couple things you've got going for you when you're being rushed. First, the enemy is (hopefully) inside your dominion so they'll have a moral penalty. Next, you've got the flexibility of levying PD in provinces you expect to be attacked - the usefulness of this varies immensely depending on the nation you're playing but for some nations just unexpectedly strong PD can thwart an initial rush. In most cases you'll have greater maneuverability working inside your territories. Finally, rushers often make mistakes in being too aggressive and storming straight to your capital, or at least deep into your territory to try and quickly cripple you. Combine these facts and you've got all the ingredients for catastrophic route pie if you can out-anticipate your enemy.
One of my favorite things to do to somebody who's rushing in too aggressively is to cut off his retreat by attacking the province he's moving from, then jack up the PD in the one he's moving to while moving several mages in to support the PD - hopefully with spells like panic or terror, in addition to whatever you need to specifically counter what's rushing you (earth meld, etc). If you do this right when they're moving out of a wasteland or low population mountain province hopefully some of them will be starving. Pull this off correctly and the enemy will route and be totally destroyed while you have absolutely no losses and turn the rush around on him as you move your army back offensively.
At least...in theory.
