How to keep the borders safe?

[HJ]

I need some advice on how to deal with long borders or multiple-front wars. On big maps, there usually aren't many choke points, and the nature of the game makes the borders very sieve-like. Obviously I cannot have an army in every province, and the AI has a tendency to bypass my armies and outlaying provinces and instead go on a rampage in my back lines, destroying my income, and forcing me to retake all that again. Sure, those armies starve, but by the time I manage to deal with them and retake everything, the new ones appear and the cycle repeats itself. In addition to that, the AI tends to recruit armies in those newly conquered provinces and buy them defense, even when they are completely cut off, which makes it harder to reconquer them. The same thing happens with multiple-front wars. If I'm concentrating on one opponent and trying to catch all his roaming hordes, at the same time another AI nation will touch my borders on the other side of my empire, and do the same thing that I was just trying to prevent that first nation from doing. So, what is the best way to keep them off my back? As I said, I cannot keep an army in every province if I'm in a middle of a big continental mass, and buying defense everywhere is expensive (and often results in crappy performance by giving me unarmored militia and slingers). What would be the best way to deal with opponents who avoid direct clash of the major armies and instead turn to guerilla tactics?