I tend to plan to attack a prime target, but also I tend to try and keep alliances and attack historic enemies. Often I end up at war with people who attack me. My recent Byzantine campaign which I won at 100% - and was well infront on GA - was over by 1315 and the only people I invaded - as opposed to responding to me being attacked - were the Muslim nations and rebels.
In war I like to hit hard, pause then eradicate. I like to King hunt for the money and to provoke civil war. I also like to bribe when possible.