Terrain is one of the biggest factors with me. I almost never cross plains, grasslands, desert, if im in a war. The AI seems to have an evil nack of always staging counter attacks at the worst possible moments
Another factor with me is opening up two fronts in an offensive war. I try to do this always, and if i myself cant, i try to get another Civ in the war that is on a seperate front.
To contradict what i just said, i also try and avoid getting other Civs into the war with military alliances as when i feel the need to call a peace treaty, i soemtimes have to wait up to 20 turns till the deal is over so i wont break it and ruin my reputation
Artillery is good to have as well, i almost always bring it alogn with me in war. Catapults less so than Cannons or Artillery.
Micro-managing my cities so they wont turn into a country of civil disorder
I always remain in my goverment of choice while waging war, as i dont generally use religious civs and the time of anarchy hurts, so i have to micro-manage as much as possible.
Hmm, what else. Using sea fareing units to carry land units over on a seperate front to cause chaos and disorganization.
There are a ton of things, usually depending on the situation.