My strategy (depending on the army) is to let them attack me first, waste their army, then I go on the offensive. Allows me to retrain troops and attack the enemy who have already lost most of the defending forces attacking me. This always works.

But, ive experianced that a good defence is an offense. Im playing Scythia and my horse archers have wiped out armies on the move. Thus leaving their settlements weaker so I can attack them. I prefer to fight them in the open cause seiges are suicidal w/ cavalry-based armies.