Lost in the mists of time was a nice little essay by Matt Finch, in which, among other things, he lays out how one might go about modifying Dungeons & Dragons Third Edition in order to make it more like old school D&D. The original post was found on ENWorld, but it's no longer to be found in the forum archives, at least not that I've been able to discover.
In any case, I'm not really interested in the specific topic, since I no longer play 3e. However, I think Matt's general advice is quite excellent, which is why I'm borrowing parts of it here, with apologies to him. My goal is simply to lay down some basic guidelines on mechanical/methodological elements that contribute powerfully to playing D&D à l'école ancien.
1. Magic items should never be available for purchase. Instead, they must always be won from opponents or discovered by exploring the hidden places of the world. In a similar vein, magic should never be a substitute for technology. Outside of the PCs, their allies, and their enemies -- and perhaps not even then -- magic and magic items should be rare.
2. Always award XP for gold. The reason for this is that it makes acquiring loot, not killing foes, the primary focus of dungeon delving. Once the players understand this, they might begin to behave more sneakily and realize that discretion is sometimes the better part of valor. XP should still be awarded for defeating enemies, of course, but it should pale in comparison to the XP gained from treasure.
3. Keep characters poor by any means you can. AD&D used training costs as a way to ensure that the PCs never had much money, as they rose in power. OD&D has no such mechanism, but I'd consider adding something like it. I know of several referees who only give XP for gold that's spent. This is a nifty idea, as it rather nicely emulates the way that Conan or Fafhrd and the Gray Mouser would go on binges of spending after they'd completed a particularly successful adventure -- only to wind up poor again in short order. Either way, PCs should rarely have a full coin purse.
4. Make the acquisition of new magic-user spells difficult. They should rarely be available for outright purchase; when they are, they should cost ridiculous amounts of gold. Otherwise, the MU must win his new spells by finding scrolls or spellbooks. The point is to keep the magic-user's repertoire limited, both to encourage creative thinking and give him yet more reasons to go adventuring.
5. Encourage map-making by presenting environments where having a map is a must. D&D is as much a game of exploration as it is of heroic exploits. I've said before that planning a dungeon expedition is a bit like planning a safari or archeological dig. Having to draw a map in order to avoid becoming lost or suffering some horrible fate is essential to old school D&D. Yes, this means the game will run more slowly, but that's an important part of the old school gaming experience. This goes for wilderness exploration as well as dungeon exploration.
6. Sometimes there are no answers. That is, there are mysteries that will remain such. Not everything has an explanation, at least not an explanation available to the PCs. It's important to remind players that there are many things beyond their characters' ken.
7. All politics is local. Old school gaming is not about world-spanning, "epic "plots and cabals. There is no single Dark Lord whose machinations are behind the rise of evil in the world. Indeed, if you feel the compulsion to talk about the Big Bad Evil Guy of your campaign -- or, worse, to use the abbreviation BBEG -- seek help immediately. Likewise, there are no large organizations of do-gooders who oppose them. Instead, everything happens on a much smaller scale, with events being confined to (at most) a few hexes on the map.
8. High-level characters are few and rare and generally have no interest in helping -- or opposing -- the PCs. They have better things to do with their time.
These are some very broad strokes. I have some more specific advice I might dispense in a future post, but these ought to keep people busy arguing for a while.