[dirtywick]

Province defense helps, it gives you free units that will defend a province, although you can't script them to do anything or set their formations. It's on the same window as the province info near the income and all that, just click on the defense and raise it appropriately.

Another thing is to keep moderate armies in the middle of your empire to cut the AI off if you're having trouble keeping them out of your empire and supplement where you think they'll strike with that army and some PD. Or you can siege one of their castles, sometimes they'll pull back to get you off their castle.