usually during sallies, the enemy sends out his entire garrison to fight you, if you would only give him a chance. Try to do as much casualties as you can before they actually rout (charging them in the rear, for example) and ensure that as few as possible get back to the town square. After that, all you need to do is quite simply swarm them with the cavalry that entered the city. Infantry need not apply. Normally only about 5% of the enemy are left in the city when I send my cavalry in because they like to rally at the square and come right out again to get routed. So it's really a matter of taking their charges until they've been whittled down.

That's the way I found to ensure the sally pursuit bug didn't affect my gameplay too much.