First of, you're going to need to find a decent village. A small one will take longer to have a big population. If you can't find a village, You're going to have to build your own, and use an inventory edit like TMI to give yourself Villager spawn eggs. Next, add doors to ALL the buildings. Adding doors will increase the number of houses. Villagers will count any door with solid blocks behind it (don't put blocks directly behind the door, or villagers won't get in the house. Put them a few spaces away. You usually don't need to do this, since the wall on the other side will probably count.) as a house. Next, make sure NONE of the doors are facing each other. This will cancel out both doors, thus giving you a loss of 2 potential houses. After that, expand the village. Add more houses, and do the previous steps with them. Next, add second floors. Of course, add doors to the new floors. This will not only add to the house count, but it has a chance to cause a glitch where villagers will breed infinitely. If I'm not mistaken, this was the glitch fixed in 1.2.5, so the infinite breeding might only work with lower versions. But still do this step even if you are on 1.2.5.

By now, you should have baby villagers running around. Note; all of them will wear farmer clothes even when they grow up. Just exit Minecraft and enter it again, and their jobs will update, giving them a different robe.(unless they really are farmers :p ) Don't worry if you are building your own village with the farmers from spawn eggs, their kids can have a different job.

There! Now you have a nice growing village. The population count can go down though. Make sure you have an Iron Golem or fences protecting them from zombies. Also, villagers can be stupid enough to push each other into holes, Cover up any deep holes they can fall into and can't get out of. Villagers can wander too far off, but this is rare, since they tend to stay together. You can still prevent this if you want by fencing the village.

Hope this helps ;)