Here's the method I've been using for a while, if you need a picture just say so:
Start on y=11, make a one block wide tunnel. Mine forward until there is no light from the start / last torch you placed, place a new one. Go back to where you started / placed the last torch. From there dig a 5 block hole into the side of the tunnel, each one you dig make sure it has a 2 block gap in between it and the last hole. Do this on both sides. Dig only into the upper part of the side, not the bottom unless there is ore inside and you need to get to it. With a fortune III pickaxe I got over a stack within an hour of mining just by doing one long tunnel. This lets you make an 11-block long tunnel (as all the ore that would be exposed by mining all the stone there will still be seen) and minimalize the use af your pickaxe. Connect these tunnels into a grid and get loads of ore. Also bring flint and steel with you to dispose of the excess stone as that will fill your inventory rather quickly.