If you have played Empire Total War you may think you are going to be great at Napolean Total War, and you may not be far off. However, there are differences in strategy needed as some game mechanics have changed drastically. Play enough and these things will become second nature.
10. Fight your own battles: As always with Total War, this rule does not change. Even the most uneven fights will end better if you play yourself; whether this means retreating or sealing off the enemy so their units are outright destroyed rather than just shattered to reappear on the campaign map. Besides, how can you get good at fighting in any situation if you let the computer handle things for you?
9. Understand Replenishment: Learn how replinishment works so your damaged units are healed before the battle begins. Also know exactly how much you will benefit from supply depots and the like. Territories that won't see combat usually don't need replenishment buildings. Your front line, however, will almost certainly need some.
8. Keep a Mobile Force: You will always need to be able to react to the enemy, but surprising him is also key. I've won a few dozen battles by destroying incoming reinforcements before they could join up with a larger force. Because I had faster units, I was able to send their friends running before they joined the battle proper. Higher mobility also means flank attacks, overrunning artillery, and many other powerful strategies are at your command.
7. Destroy the Enemy: Shattering the enemy is nice, and removes them from combat, but when possible, decimate the enemy units so that they do not reappear on the campaign map. The worst case is loosing your army to an unfortunate accident after shattering a number of units, and having the entire enemy army reappear and reorganize while your army is destroyed. Napoleon makes this harder by removing the flags from the shattered units, but it is still fairly easy to keep track of enemy units that are fleeing.
6. Seek Targets of Opportunity and Annoyance: Damaging things in enemy territories with small forces is a good strategy to annoy your enemy into revealing his forces and in general just wasting his time and money. In battle, targetting isolated units, creating advantages, and a whole host of opportunistic events are available to the player. Use these to your advantage.
5. Bull Markets: Your counties are worth a lot in this game, as are their economies and buildings. These will ultimately win the war, and no matter how many battles you win, economic attrition will eventually topple you if you are not careful. The economy is the long-term solution to the war.
4. Create a Balanced Army: An army should be able to react to any situation, and work well together. A mix of artillery, cavalry, and infantry should do the trick. More or less of each according to taste, but you will find yourself wishing you had one or the other when you are lacking them. I will note that units don't get along as much with artillery in this game, and you will kill many of your own units if you are not careful with them around your guns.
3. Have a back-up plan: Things will go wrong. Plan for it. Always be building extra units, have another strategy, find a way around. Inevitably, the enemy will outsmart you somewhere, and you will find yourself rushing to your own defenses. If you are ready for anything, you can quickly turn the tide on the enemy, and turn his gambit into a fatal move.
2. Make Use of Terrain: In the campaign and battle maps, terrain is important. Cannons like hills, infantry like open line of sight, cavalry operate best on the open field. Know your units, know the terrain, and you will win many battles. Also on the campaign map, you can utilize chokepoints, plan invasion paths and the like just by careful examination of the terrain. Also, the two intertwine, so if you have a lot of cavalry, fight on/near the plains in the campaign map to have a smoother map when the battle starts.
1. Understand Attrition Warfare: Napoleon is very much an attrition campaign. The changes to the replenishment mechanic, and the style of warfare make it obvious. You will loose troops, and troops that survive will be severely depleted. New troops will always be needed and needed on the front lines. Get used to this style, and realize that you will not always have veteran units, and the units and mix you have now will probably be much different after the battle. Make the best of it, and ensure that your enemies suffer the most. After all, an army shot to pieces is preferred to an army that ceases to exist.
I hope you have fun in Napoleon, I know I will be having fun with this game for a while.