Also, the game is generally rather unfair towards the defender. Sure, you get the defence bonus from territory, rivers and such, but you are forced in a passive role, and what's worse: You can't do anything while you are being attacked. Often times, you will lose a defensive battle, that you could have won handily, were you allowed to counter-attack, because of the encirclement bonus. This is why I believe that attack is often the best defense.
If you HAVE to defend, you should look for good defensive positions. One thing that makes a big difference, is to make sure that you province can only be attacked from one other province, to deny the enemy a potential encirclement bonus by attacking from multiple provinces. Sure, this is not always possible, but it helps.
And then, generally, if you can pull it off, try to defend along a line of one corps per provinces, don't stretch them out and build corps that slightly overfill the maximum width (say 13 or so), basically to have one spare division, one more division that can participate in the initial line up.