To find dungeons easily you need to block yourself in a little room that you can fit in (better 2 blocks high and 1 block wide). Press the F key or the fog key you set to if you did in the options and rapidly press it. It should make everything but your hand disappear for 1 second. Every time the terrain disappears look for a monster spawner. If you see a monster spawner that's a dungeon. Note: If you are playing version 1.8, if you're in an abandoned mine it can be a cave spider spawner.