concentration of force...hit a flank with your whole force to attain local superiority of numbers, while 'refusing' with the other flank.
the wikipedia page on Epaminondas and his victory at Leuctra has a good description of this, he is credited with inventing the right flank refuse.
http://en.wikipedia.org/wiki/Battle_of_Leuctra

also sometimes the AI will attack you even when it's nominally the defender if it has a superior numbers, then you can pick your ground.
if you have skirmishers you could try enticing the enemy general out and cutting him off.