Over on Delta's D&D Hotspot there is an article (with a PDF full of spoilers) with tactics for the enemies in D1 Descent into the Depths of the Earth.
Planning the enemy's tactics ahead of time is an old idea but a good idea. If wasn't for the tactical advice in A3 Assault on the Aerie of the Slave Lords I don't now that I would have known how to handle such high-level foes when I first ran it.
Leaving complex powers and in-depth tactical planning to a game-time decision isn't really such a good idea. It's too easy to remember too late that, oh yeah, the fire-men could have started chucking fireballs at 100 feet away and not wait until the PCs moved up, or that the drow had special detection abilities, or that the trolls should have driven their pet lizards ahead into melee while they move to the flanks. This is especially true for intelligent enemies. If the players are going to carefully hone their tactics just to survive in a hostile world, it's odd that the NPC party they encounter isn't doing (or didn't need to do) the same thing.
Here are some things I've done for this situation.
Have a rough idea - You don't need a hideously complicated plan. But you do need to know what the enemy will do first, second, and third. You'll need to know how they'll react to (likely) counter-attack by the PCs, or to specific attacks (i.e. they'll run from fire, or use their buckets of water).
Map It Out - Either on a battlemap if you use one, or on a piece of scrap paper, just sketch out what the enemies will do and where they'll be if alerted and if not.
Look it Up - Check on the enemies' special abilities, spells, or weaponry and make sure you understand how they work. This alone will prevent a lot of "oh, they could have just done [x] and avoided that whole disastrous defeat" issues.
Test it Out - This takes some time, but if the enemies supposedly have been doing this in the past you're going to want to detect any major flaws by giving it a try. If the hobgoblins guard a chasm crossed by a rope bridge and have successfully defend it for years, it's unsatisfying to find out you left them wide open to a simple area spell or left cover on the far side that nullifies their weapons. Give it a trial run, either against the PCs (if you're trying to get a specific challenge for the party) or against some generic foes they'd likely be used to fighting (if you're just probing for flaws.)
Beta-test it. Give it a going over.
Know if they're coordinated or not - If the enemies fight in coordination, great, know that and plan around it. If they're essentially chaotic and disorganized, know that too. Just because the bad guys aren't very coordinated or clever doesn't mean you don't need to think about it. We're talking about your plans to make your life easier as the GM, not saying that all enemies work in a coordinated fashion.
So maybe the orcs charge wildly while their supposed ally wizard throw area-effect spells on top of them, or one of the enemies runs forward screaming his battle cry. But it's worth knowing ahead of time what they're trying to accomplish even if they all have their own plans.
Know their reinforcement or fleeing options, if any - If there are more friendlies nearby, how do they summon them and when do they arrive? A force roster is your friend, here. If they have to run, where will they run to?
Get an Adversary Player - One way you can really make encounters more interesting is to get someone else uninvolved in your game to develop the enemy's tactics. This can really give the encounter a different flavor because the enemies react less like you, and instead act like someone else. If you've consciously or unconsciously got a tendency to deploy certain tactics or make certain moves, it's really surprisingly to the players if they encounter tactics with a very different mind behind them.
To top it off, if you're writing for other people - for publishing or just to post up somewhere - how you visualize the tactics of the enemies can make fascinating and inspiring reading. You don't want your readers to get stuck wondering "Why the heck do these guys do this?" or "What were they intended to be doing here?" when they look at your encounters.