As the north, you will generally have more infantry and artillery at your disposal, as well as a larger starting force. If you have the artillery, it is generally to your advantage to blast away with it and hamper your opponent with small attacks of 1 cards to limit his ability to fire artillery at you and also try to spot his weaknesses.
If early in the game you have little or no artillery, lots of infantry, and aren't loaded down with five or six generals and a ton of weak specials, attack! At this stage of the game, you are undoubtedly in a superior position as the south only has 83% as many cards as you. If you wait until the middle of the game, this percentage will increase (especially if he plays one or more railroad cards) and your advantage will be lost.
As the south, you will generally have to play a more defensive game until you can build a large reserve that will give you some options. The only time when this will not work is when you have NO artillery and your opponent is blasting away at you turn after turn. This is a sure-fire recipe for a loss.
Again, you will have to mount an attack on the artillery and hope that you can knock some of it out. If you succeed in hitting two artillery units and your opponent replaces them with infantry, don't feel there is some dishonor in retreating. After all, you were trying to knock out the artillery, not commit suicide against a numerically larger force of infantry. Force your opponent to attack in that situation. You'll get the first shot, and given that you are the south, there is a built in tendency for the north to avoid a melee as you can always pull out the "Rebel Yell" card.
When to Attack and How
Breakdown of "Special" Cards
Strategies for Each Side (this page)