I think that fast victories are the best way against big (in size) nations.. To have a successful war the following tactics might be useful (some are mentioned before I think):
1. Destroying enemy devisions (early game)
-In the early game, the fire values of armies are still low. This results most of the time in battles where you or the enemy flees before you (or the enemy) can destroy devisions. So having a higher morale then the enemy (choosing Jominian attitude) is very useful.
-The ai has most of the time no real manpower problems, so even if they lose a lot of troops, but not their divisions, their army will stay intact because they keep reinforcing
-So the best way to destroy the army of the enemy is to encircle them. If all provinces around an enemy army are occupied by you, the enemy devisions will be destroyed if they try to retreat in a battle. In this case the enemy will try to fight to death
-You could try to lure an enemy army in your own country by leaving a province without troops. Then reconquer the surrounding provinces (don't send troops to the enemy army) and then when they are encircled, attack. Of course you could also encircle the enemy in his own lands, by sending your army to occupy provinces around the enemy before attacking him (Most of the time I only attack enemy devisions if they are encircled or if I can encircle an other enemy army by taking a province defended by the enemies troops)
2. Speed is important : D
-If you are faster than the enemy, you are more likely to have chances to encircle enemy troops and occupy the enemies provinces faster than the enemy can reconquer them back. Also if you win a war faster, partisans are less likely to show up.
-So to be faster than the enemy, it can be useful to use some cavalry in the early game, especially because cavalry has also a high morale and a high shock attack (they reduce enemy morale fast)
-Also try to occupy the province your army is located, before you move to another enemy province (if the province you are standing on is not yet in your grip, your army will move very, very slow)
-If your enemy has a large coast, it might be useful to transport your troops by sea instead of letting them walk. So send your troops aboard when they have captured a province and send them ashore to the next coastal province. (especially useful if the coastal provinces have a mountainous terrain). But keep an eye on your troops to avoid the enemy encircling you, and make sure your transports are defended by warships.
3. Capture the capital
-If you occupy the enemy capital you gain 20% warscore, so by holding the capital you can quickly get a favorable peace.
-If you plan on a really fast war, your goal should be to rush your troops as fast as you can to the enemy capital, but avoid encirclement of your troops

A navy might be useful for a landing if your enemies capital is near the coast.
4. Look out for partisans
-If you think the war will take a lot of time (if you fight in jungles, mountains in a big country), try to leave a devision behind for every 2-6 captured provinces (if you have enough troops). If partisans appear you can then quickly send one of these reserve troops to kill the partisans, before they can reconquer lands behind your front.
5. Terrain is important
-The defender can get very high combat modifiers when fighting in mountains or other hard terrain. You can easily lose even with 10-1 odds when this happens. A way to attack in these terrains is by attacking from multiple directions (every direction gives you an 10% bonus) and encircling the enemy.
-Sometimes the best way to win against an enemy in bad terrain is to withdraw your surrounding troops, so your enemy moves to another province, giving you the chance to kill them or just occupy that province.
6. Other important things
-Technology can help a lot when fighting.. If your enemy has more technology you might need 2-1 odds sometimes and you'll lose more manpower, so try to have a better tech if possible
-Have a bigger army (very important

)
-Having brigades might give you an edge over the enemy (but they can be costly)
-Having a (small) navy with enough transports and maybe a few frigates (or iron warships, if your enemy is advanced) can help a lot when fighting against minor powers (in South-America) to have more mobility and the option to fight really weak powers (Haiti, Uruguay (if you don't have a land border with them))
I hope you find these tactics useful and not too hard to read, also some of these tactics are only for the early-game
Good luck with the warmongering
EDIT: hmm the wiki-link above mentions also a lot of this I see now
