I've been refereeing the Dwimmermount campaign for a little over two years now, but planning for it began several months beforehand. Part of my planning included giving some thought about what house rules I wanted to make to the LBBs, the results of which can be seen here. Looking back on that post, I feel a small amount of embarrassment, because I can see that I fell into a trap that I've often cautioned against in other posts of mine, namely, treating OD&D's rules as something distinct from their actually being used in play. Of the rules modifications I listed in that entry, most of them have never been used or, if they were, they were quickly abandoned. I use the alignment system (though it has minimal mechanical effects), the hit dice re-rolling, liquid courage was used briefly at 1st and 2nd levels, and scrolls (also less used now than they once were). The others have all come up in play from time to time, but, more often than not, we simply forget them.
What this suggests to me is that, like so much else, house rules are usually better if they're a consequence of having to deal with a genuine problem that crops up in play. I've noticed that lots of gamers will claim that rules X or feature Y of OD&D "doesn't work" or is "broken" and sometimes they're right. But "brokenness" is situational and what one groups finds problematic, others will not. Likewise, many referees -- I know this, because I am one of them -- become obsessed with "fixing" this or that or introducing some mechanic that they think is neat, only to find that the fix is intrusive or that his players couldn't care less about the mechanic (such as my early attempt to introduce a weapon vs. AC modifier table à la Supplement I).
My point is that, unless one has extensively played a particular system with the same group of players over time, creating house rules in advance of actual play is jumping the gun. The most durable -- and useful -- house rules I've created for the Dwimmermount campaign are those that were created, often on the spot, to address a problem that arose at the game table. The most forgettable -- and useless -- "house rules" I've created are those I came up to scratch some intellectual itch of my own. Consequently, I've largely stopped tinkering with OD&D's rules in the abstract, which is why I so rarely make game mechanical posts to the blog anymore. The ones you do see are things I either used in actual play or that I created in response to an immediate (or at least imminent) need in the campaign. I won't say this is the only approach, but it's definitely my preferred one these days.