mmm... i don't really remember it as a big problem, but i'll try to think of what i do.
hug a wall. if you can't avoid the lance completely, put yourself directly in front of the horsre and let him run you over. practice horses don't do much charge damage, and odds are your opponent will be stuck on a wall afterward so you can kill him or his horse.
note that if you don't do anything except run forward for a few seconds, you will start running faster. this can make it easier to position yourself properly.
if you've got a shield you can eat the lance with your shield and proceed to the beat down. parrying down with a melee weapon will stop a lunge but not a couched lance.