Rhino Hide Armor: +2d6, not that sexy
Impaling Weapon Property (MIC): 3/day as a swift action, next attack is a touch attack. Not more damage per say, but makes full PA with Leap Attack much easier.
Cavalier prestige class (CW): Your smite damage will stall, but gets to deal x4 charge damage a few times a day, and eventually x5.
Steadfast boots + Cometary Collision (MIC; PH2): boots let you set vs. charge for double damage with any weapon; feat lets you ready to intercept a charge with one of your own, simple enough. As with many possibilities, debatable if the mount needs the feat as well.
Elemental weapon crystals (MIC): For you, the least are actually a good deal, since that 1 energy damage x4 is better on average than a lesser crystal's 1d6. It's small, but it's something...
Leading the Charge stance (ToB:Bo9S): One level dip into Warblade or Crusader or two feats gives you ~ +1/2 HD to your charge damage, as well as all allies within 60 ft. Multiplies, of course. Dipping might be the better option, because...
Battle Leader's Charge maneuver (ToB:Bo9S): Maneuver adds +10 to your charge damage (multiplies) and you do not provoke AoO's for the movement. Can be had with a level 1 dip, buying it with feats is more of a pain in the ass. Dipping also means you can re-use it in the same combat.
I'm sure there's more, but it's late.
EDIT: I just assume people upgrade by that level, but I noticed you're still using a medium lance.

MIC, Strongarm Bracers. For a mere 6,000 gp, you can use a large lance, base damage 2d6 (on average, +2.5 damage before the x4 multiplier).