The standard XP for treasure and monster-slaying, with an emphasis on the treasure, isn't too bad. After all, the rather mercenary, picaresque nature of D&D comes from swords & sorcery in the first place, which is why the XP system was designed the way it was.
But not all swords & sorcery is about thievery. In between Fafhrd and the Grey Mouser pulling criminal jobs, there's brief stints of religious devotion or weird supernatural menaces. Dilvish the Damned doesn't do much thieving at all; he's on a quest for vengeance, with a slight detour to rescue his old homeland and several interruptions by his own supernatural challenges.
What I think is a common thread in most of the stories is not treasure, but service. S&S heroes are usually working for some employer. In some cases, there's a reward; in others, there's an obligation. In some of the jobs, the goal is not theft, but disruption of some organization: payback against the thieves' guild, overthrowing a baron, attacking the slavers. This could even be applied to personal quests for vengeance. Other times, the hero is working to keep a group intact: defending a country (sometimes by raising an army,) restoring a king to the throne, helping a church out of a tight spot.
What S&S heroes should perhaps get XP for is the reputation they gain from serving a powerful leader, disrupting the service of another leader, or earning such service themselves as a leader.
Rate each leader or organization according to its size. How many six-sided dice would you need to roll to get the number of followers that leader has? That's the "hit dice" of the leader.
- Serving a leader net you a little experience: multiply the leader's "HD" by your own Charisma.
- Injuring the reputation of a leader or organization and getting away with it nets you 20 XP per HD.
- Acquiring loyal followers of your own also nets you 20 XP per HD, plus 1 xp for every follower your followers have.
Low-level heroes will often work small jobs for small leaders, getting a little XP from each. Mighty heroes build their own organizations; the mightiest usurp the throne.