Ore distribution is even. Most people want to be mining in the diamond layers and Layers 12-14 are pretty standard to be used for this. Layer 12 is where most lava pools spawn, thus the logic in mining at that level or above is to avoid lava constantly falling on your head. Plus breaking out into an open area above lava pools shows more exposed stone than you would see simply by digging a shaft in a straight line through solid stone. The reason for the 14 level cap is because diamonds go up to layer 16. Thus if layer 14 is the layer you're standing on, then you're body is occupying layers 15 and 16. Mining above there you're wasting time by mining half outside the diamond layer.
Now with all that out of the way, the 'best' mining method with those constraints is mostly about trying to maximize the amount of stone you're exposing in a set amount of time. The easiest way to do this, is to not waste time you're in the mines by walking up and down your halways as you get to your next branch. All that walking around down there is just eating up time you could be chewing through blocks with a pickaxe. So in order to minimize the time you're wandering around, your mine system needs to be compact. People tend to create a single 'central' tunnel extending in one direction and build short branches back and forth off of that. But they forget, there are 4 directions you can be mining in! Why waste all that time going in a straight line? Ultimately you'll have to walk all the way back!
Some time ago, I came across http://www.minecraftforum.net/topic/285 ... dable-v02/ on the mcf forums. This is the method I personally use. The link explains how it works better than I can.
Over time, this is what the mine starts looking like. This is a rendering of one of the layers of a mine in one of my ssp worlds.
Spoiler
Show
I can add more mining methods to this main post if you've got something you use and works well.
