To begin, initial placement of your cards can go a long way to helping you succeed. First off, you need to figure out the best way to allocate any terrain cards that you have received. While terrain always helps, it's important to remember that it will limit the number of cards that can be placed in a position. Thus, even if I have three or more terrain cards to use, I will generally keep one space clear, except in certain situations.
Woods are the best terrain for defense. Hills are the best for offensive artillery at long range and defense in general.
The creeks are good for limiting an attack and can come in handy if you are undermanned as the south often is. This is the situation when you are best advised to play all the terrain you can. Place a creek card on one of the flanks and protect it with one or two light units. The creek will limit an attack on your flank, and even if your opponent has a ford card to play, will limit the size of any attack to two cards, three if combined with a flank. Place any other terrain cards as you like. I think that the woods are the best as they reduce the CV of attacks, especially long-range artillery. Hills are beneficial only in that when attacked, you get one extra CV each, other than long range artillery considerations.
INFANTRY, CAVALRY, and GENERALS:
Once you have placed the terrain, you must look at the rest of your cards and design an initial strategy. If you have lots of artillery, place one or two of them in any woods terrain and keep the rest in reserve. Allocate some infantry throughout your positions, keeping several of the big ones in your reserve as well. If you are short of manpower (usually the south), try to keep at least one powerful card in your reserve. It will come in handy! As to generals, don't bother placing "attack" generals (those who assist morale rolls when engaged in the opponents ranks) but be sure to place good defense generals whenever possible.
Once play begins and terrain has been revealed, place the bulk of your artillery, preferably on a hill. Then start blasting away. However, don't shoot into the woods. If you have artillery that is facing your opponent's woods, bring it back into your reserve and place it so it can attack somewhere else at full strength. A hill opposite woods becomes a wash, with the added problem that attacking ground units that follow a successful artillery barrage will still have their strength reduced by 1. When both sides are relatively evenly matched in artillery strength, this is the conservative strategy that generally prevails. Keep shooting the artillery and weaken your opponent until you see a problem develop that can be exploited.
General Strategies (this page)
When to Attack and How
Breakdown of "Special" Cards
Strategies for Each Side