On the tactical level, if your army is roughly the same size as the enemy, it should take a little while to kill you off in a particular battle. You can retreat as soon as the mandatory fight period is over and preserve your army for a more opportune battle. Of course, if you are fighting to preserve your core provinces, that may not be an option. Better is to position yourself to have the tactical edge before war starts--I try to only attack clans the same size as myself only once they are involved in a war with someone else and tire themselves out. Before that, I gift them and send them my daughters to keep them friendly while I gobble up the smaller clans all around. (They almost never refuse a daughter bride.) Keep checking back on the plots, people change their minds frequently, especially if your enemy is big. One the fighting starts, if they have a commander that has a very high martial rating giving you problems with +3 or whatever, see where he lives and have him assassinated by ninjas. Another ninja strategy is to lower your enemy's clan leader to below 10 to cause him all sorts of vassal problems. If there is a lot of provinces changing hands between you and him, though, that may not be worth the cost because he'll get honor quickly back.
In my current game, my main enemy is at 45% and I am at 35% and slowly climbing against him. I've started getting the feel for how his forces move in this very fluid war model. It helps to very frequently check where your enemies' armies are heading because they can change very frequently. One thing that I do that helps is frequently go through your levies and see if any are really low (less than 500) and would be at near full strength if you dispersed it and reformed it immediately back home. Also, make sure your vassals each have 4 or 5 provinces so they get larger personal retinues.
