My two cents. When playing cav, always take a heavy lance (higher thrust damage). Second, couching in multiplayer is basically only useful when charging in formation, backstabbing someone, or to break someone with a short weapon's shield. Bottom line, use couched lancing SPARINGLY!
WARNING: NEVER TRY TO LANCE DOWN ANY OPPONENT WITH AN AWLPIKE, LONG AWLPIKE, PIKE, OR ANOTHER LANCE. (These weapons are too long, and head on will shortly allow you to meet your maker.
The trick to killing infantry..
Does the infantry have a spear?
If yes: Wait for a back stab opportunity or try to thrust from the side of your horse.
If no: Couch, charge, and be sure to reach full extension of your lance as you "drive by."
I had made mention of thrusting from the side of your horse, allow me to attempt to clarify. When you charge someone your horse is in front of the rider. This is a huge problem, as your horse will most likely be hit and killed before you can hit your target. To eliminate this, and to get more reach from your heavy lance, simply, thrust from the side of your horse (this allows longer reach and is a trick most pros use).