The overall theme for all my house rules is K.I.S.S. The lighter-weight the better, keep it fun and fast moving.
So, for XP, I go with the 100 XP per HD old-school ODD guideline. Simple to keep track of, simple to calculate, and gets our heros moving up relatively quickly. Especially in the context of not playing very often. If I'm ever back to having summers off and playing for hours every day, the more detailed XP calcs and harder-earned progression would be appropriate, but when you are only playing a couple hours two or three times a month, come on!
I am also a fan of the 1 GP = 1 XP rule. I put a slight twist on it, because I require the PCs to "go to school" to level up. Going to school requires tuition, right? So it is a built in requirement that PCs must acquire some treasure in order to be able to afford the tuition at their academy to progress in their career.
I set the tuition amount in GP at about 1/2 of the XP required for that level. For example, if She-Ra the Half-Elf magic user needs 2500 XP to level up to 2nd, the tuition for her training will be at least 1000 GP. So, as a newly minted 1st level witch fresh out of ASU (Applied Spellcasting University) in Tempe, she knows she can't level up without getting her hands on a pile of cash. Dangerous adventuring beckons! I mean, really, where else is she going to get that kind of money? The Wizards Guild is paying 1000 GP per vial of dragon's blood, hmmm, maybe that is a good place to start...
As the "points of light" are so spread out due to the Dawn of Civ setting, the training locations in Arizona Advntures are fairly limited and centralized:
--The good magic users are trained at ASU, while the evil ones attend NAU (Necromantic Arts University).
--The noble-minded fighters, cavaliers, and paladins are trained at the King's academy in Tempe
--Rangers are trained at the Wilderness Survival School in Wickenburg.
--Lower-class, neutral, and evil fighters find welcome at the rough frontier academy in Prescott. --Aryan clerics learn their trade under the high priests in the Tucson Temple, while Monmore clerics apprentice in Mesa.
--Druids study with the Celt wisemen at Fountain Hills.
--Thieves have a guild for further training available in either Tempe or Flagstaff (assassins in Flagstaff only).
Of course, going to school only works up to a certain point, probably level seven. After that, it is assumed that characters are progressing through their own hard-won experiences in the school of hard knocks.
P.S. It is also my practice that XP can be earned without "fight to the death" encounters. As encouraged in OSRIC, losing opponents will disengage and flee, and PCs earn XP by defeating monsters, not just for slaying them.