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[Yair]

I have had bad experiences with requiring downtime in D&D games, but then I wasn't after what you are.

Have you considered divorcing XP from adventures alltogether? The mage could gain XP by studying forgotten tomes or from demons/celestials/outsiders/dragons, the fighter by participating in wars or picking up techniques from grandmasters wuxia style, the cleric will gain levels as blessings from his deity, and so on; they will adventure to seek out new sources of enlightment (and levels) rather than to gain fame and riches, and will gain levels only when it is appropriate story-wise.

I find the entire XP-level model problematic, and have had great dificulties reconciling myself to it. For D&D, my favorite solution was to make the PCs into awakening gods, making XP mark their growing remembrance of their divine selves. But that's really taking the thread off to a tangent.
(On another tangent, the best system for modeling training I've found is in Ars Magica.)

 

[Raven Crowking]

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[the Jester]

As sad/lame as it may sound to some....

I am actually developing rules for down time. Specifically, I am encouraging PCs to take downtime, both in order to fulfill requirements (research/craft/training/etc.) and to rest. I am trying to break the "1st to 20th in two game years" skyrocket by allowing seasonal turns wherein the PCs rest, recuperate, and take care of minor business.

I also intend to allow an XP cost in lieu of training. "Training on the run," so to speak.

Hmm, I've done something much like this in my low-magic ruleset. It goes like this:

Training: Characters must spend time to train when leveling before acquiring skill points, feats, class abilities or spells. Increases in hit points, save bonuses, base attack bonus and ability scores due to character level are gained immediately (without training or rest). A character may either train under a master or train herself. As soon as a character gets enough xp to gain a level, she gains any appropriate increase in hit dice, BAB, base saves and caster level (though not spells known or spells per day). If the character has a familiar, animal companion or special mount (or other creature of similar nature) it likewise gains some increases without any training.

A character may study under a master to train provided the master has any feats being gained, at least as many ranks as the training character will have in all skills being raised, all spells and class abilities being gained. Training under a master requires 1 week per level being gained (so training for 2nd level requires 2 weeks). Some masters will charge the character being trained in money or other compensation.

A character training herself requires 2 weeks per level being gained. She must also spend 100 xp per level being gained to train herself. If the character does not have enough xp to pay this cost without losing a level, she still pays the cost and takes the level, but she also acquires a negative level that cannot be gotten rid of until her xp rise to the threshold for her current level. (This represents physical and mental fatigue from the efforts of self-training.)

I hope you find this useful- the one flaw in it is that the group I run doesn't always level together. :\ That, combined with a mostly low-level propulace, means that the group spends a lot of time looking for training. (I think that will self-alleviate as the group moves into the big city though.)