These videos show you how to build a fully automatic and expandable iron golem farm using a (fake) village. The position of the doors and floors is criticial; the farm was designed to allow two spawning floors for efficiency.
With roughly 10 golems per hour, the yield may not be high (considering the required space), but, to my knowledge, it is almost as high as it can get (the spawning floors could be optimized so that they're on one level, i.e., no lowered central stream). The only way to further improve it would be to build several farms and a shared item canal (repeat the design every 72 blocks or more to be safe).
The farm needs about 20x20x18 space, plus some overhangs. Skylight is required for some blocks. Also, a village close to your desired farm spot would help a lot.
F3 Mod (Village Info)
In the video, I used a mod called Village Info. It shows some basic info about the village you're currently in (or close to). The mod helped me a lot in designing this farm (actually, it was the very reason I created the mod). You can download it here: Village Info