I always have a unit or 2 of light cavalry to mercilasly cut down any routers - Numidian Cavalry Mercenaries are my fav unit for this. Always go for the furthest units first, then swoop back to kill the rest so none escape - take no prisoners!

I also charge the enemy when the AI attacks but then on the battle map decides it can't win and withdraws. I use my light cav to charge into the rear of enemy units, picking them off one at a time while the other units continue to withdraw. Again Numidian mercs are ace for this as they can give them a volley of javelins before plunging into the rearmost ranks! Ha ha