Here's a few screens I found of what was fondly named the "Zombie Rehabilitation Center". During SMP games, we'd explore around villages until we found a nearby zombie spawner dungeon. We entered the dungeon, slayed the zombies, took the loot from the chests, and put torches on the spawner cage to temporarily disable it.
From there, we built a short staircase leading from the spawner towards the village. At ground level, we erected a containment facility using gates, iron bars, levers and redstone. Another room in the facility held components for making and storing splash potions of weakness and golden apples.
Once everything was in place, we ran back to the dungeon and removed the torches from the spawner, leaving just one or two torches in the corners of the room to keep other mobs from spawning (you DON'T want a creeper to pop up unexpectedly here!). Zombies then spawn immediately and climb the stairs looking for you or the nearby villagers.
Most of the time, the "cage" portion of the facility looks like the image to the right. You mostly get regular zombies, which you can either kill for XP and loot or drop them into an optional lava trap below. What you're really waiting for are those rarely-spawning zombie villagers.
|You're looking for these guys.|
Instead of using a number of other tricks (that we personally consider cheap/cheaty) for quickly building a villager population, we'd "rehabilitate" zombie villagers using the potions and apples. It is said that iron bars increase the chances of a successful transformation from zombie to villager, thus we used them as our main "containment area". Every attempt for us was a success, and we recreated this several times on different games.
Once we restored a villager from zombiehood, we'd check their trade offer. If it looked promising, he was sent out an iron door that led to a hallway with a minecart and track, which he had no choice but to hop into. Our new salesperson was then sent to our specially built market stalls and was given his own place to set up shop - complete with a sign hinting at his best offer! The other villagers were sent out sans minecart to go join the residential area of the village.