(Edit: The core idea came from Telecanter's Receding Rules. A lot more is covered there than merely healing rates, so go check it out if you haven't already.)
Anyway, the basic idea is that the comfort of characters affects their rate of healing, and comfort includes such things as entertainment as well as food and drink, lodgings, etc. My contribution to the cause is to codify these factors into a very simple system, utilizing a slight modification of the standard B/X ability score modifier table.
First, the "comfort score" is determined, starting with a base of 11. This assumes reasonable shelter from the elements (could be a house, an inn, or a sturdy pavilion or yurt), a basic bed such as a cot or straw pallet, adequate warmth, and basic food and drink (bread and cheese, porridge, or similar, plus water or other beverage of ordinary quality.)
Add 1 point for each of the following that applies:
- Comfortable bed (soft mattress, clean linens, etc.)
- Good food and/or drink (Hearty fare, i.e. not simple gruel or bread and cheese, nor preserved rations. Roast fowl or a chunky meat stew are enough to qualify.)
- Abundant food and drink (Everyone can eat their fill, and then some.)
- Heartening entertainment (Music or storytelling or whatever else seems appropriate.)
- Care of a healer (No more than four patients per healer.)
- Full rest (No other significant activity for 24 hours.)
- Hygiene: Opportunity for bathing, washing, and grooming.
- Creature comforts (Robe and slippers, tea, incense, or whatever else makes a character feel really comfortable.)
- Pleasing environment (Neat and tidy indoor area with appealing decor, or an outdoor place of great natural beauty.)
Subtract one point for each of the following that applies:
- Inhospitable climate (Hot or cold, rainy.) (Negated by rudimentary shelter and/or campfire, if appropriate.)
- Very inhospitable climate (Bitter cold, sweltering heat. heavy rain, hail.) (Cumulative with the above. Negated by full shelter and/or fire, if appropriate.)
- Sleeping on ground
- Sleeping on hard, rough surface (A dungeon floor, for example. Cumulative with the above.)
- Iron rations (Unappealing preserved food, e.g. hardtack and salt pork.)
- Short rations (Stretching rations farther than they're intended, or living on foraged food.)
- Interrupted sleep (Guard rotation, or actually being attacked or otherwise disturbed.)
- Frightening/unsettling environment (Haunted, cursed, inhabited by monsters, etc.)
- Unsanitary conditions (Muck, slime, corpses, horrible smells, etc.)
Base rate of healing is 1d4 points per day, modified by the "comfort score" as if it were an ability score:
Less than 3 -4*
9-12 No adjustment
19 or more +4
* No spell memorization/restoring spell slots possible
** May not memorize or restore slots for highest two spell levels known
*** May not memorize or restore slots for highest level of spells known
Penalties may reduce a roll to zero or less, in which case no healing takes place and the character's condition may actually deteriorate.
Let's say our intrepid adventurers have had a rough go of things in an underground crypt, and barricade themselves in a dead-end chamber. The place is pervaded by a deathly chill (-1), they're sleeping on the ground (-1), on a hard surface (-1), on iron rations (-1), interrupting their sleep to keep a constant watch (-1), and in a very unnerving place (-1), for a total penalty of -6. That comes out to a Comfort score of 5, which is a -2 penalty to heal. To bolster their flagging spirits, they break out the bottle of fine vintage brandy they found in one of the crypts, which adds +1, making their overall score a 6. Their healing penalty is only -1, and their spell casters can recover all their spell levels normally. (If they hadn't had the brandy, they could have risked the noise of singing a few heroic ballads to achieve the same effect.)
Once they make it back to town, they spend a week recovering in the luxurious villa they prudently purchased with some of their previous loot. They have plush feather beds (+1), excellent food prepared by the villa staff (+1) in plenty (+1), they have nothing to do but rest and relax (+1), take hot baths (+1), enjoy all the luxuries of the well-to-do (+1), and in a very pleasing environment (+1) for a total Comfort score of 18.
If desired, the Comfort score can be adjusted individually for each character's Constitution modifier; thus, hearty souls can recover quickly even in less than ideal circumstances, while the more delicate require greater ease and comfort to restore themselves.