Granted, it's not a good general-purpose solution, but IMC we simply changed it so that the Mount became more like an "animal companion"; it could be as small as one size less than the Paladin, which'd obviously prevent it from being rideable. We had a Halfling Paladin with a Dire Weasel mount; he COULD ride it, but in most battles he wouldn't.
Other solutions I've seen:
> Replace the mount with a simple cohort (as the Leadership feat, minus followers). I'd limit the cohort to being another Paladin, creating a Jedi-style master/apprentice system. (If you allow the cohort to be any other class, then it tends to take the focus away from whatever other group member overlaps the cohort's class.)
> Better spellcasting. Little things; instead of caster level being 1/2 class level, just equate the two. Add a few more spells to the spell list. Give one more spell slot per day at each spell level. That sort of thing.
> Improve the "Lay On Hands" ability. For instance, you could simply increase the number of points they get; currently it's (level x CHAmod), which is never a really huge value, so you could just double it in exchange for the mount.
Or, you could then say that instead of X points of healing, the Paladin could choose to manifest certain spell-like abilities when they lay hands on someone.
5 points gets you a lesser restoration. 10 gets a remove paralysis or delay poison. 15 gets remove blindness or remove curse, and 20 gets neutralize poison, break enchantment, or restoration. All of these are on the Paladin's spell list already, this just lets them use it more spontaneously. And, you can mix it; maybe you want to heal for 8 AND restore a few stat points, for a cost of 13 points of healing.
> Have the Paladin's sword be magically bonded and aware (use the Psicrystal rules); no extra combat bonuses, really, but a few extra little perks. If the weapon is lost/destroyed or if you find a better one later, allow the "crystal" to freely move to the new weapon without penalty.