What's the most efficient mining pattern?

Personally i'm using a grid of tunnels. The vertical tunnels are using every 5 block a wooden platform to jump up or down.

  |L|  M  |L|
 _| |_____| |__
 _ L _____ L __
  | |   M | |
  |L|     |L|
  | |     | |

• L = Wooden Platform (you can jump through up or down)
• M = Minerals

the best pattern is running through natural caves, that is until you get stacked.. then you mine your way the the next cave..

Like Joe, I also use the grid of vertical/horizontal tunnels techniques.

Horizontal shafts are 3 blocks tall with periodic dams to minimize flooding if I pop an underground lake. My horizontal shafts are roughly 300 - 400 ft apart (using the depth meter) and line up with where the background textures change (dirt -> rocky dirt -> all rock.)

Vertical shafts are 2 blocks wide with wooden platforms every 6 blocks up to act as a ladder. I space them apart by counting aloud between 10 to 15 seconds while running in the horizontal shafts.

Digging the tunnels takes a long time but it helps if you craft a good pick (demonite or better) and/or get some friends. Be careful about digging though the dungeon (skulls) and flooding your tunnels with the oceans on either edge of the map. The fire imps in the underworld also make tunneling a PITA. I suggest building your bottommost horizontal tunnel right above where they start spawning.

I guess this isn't really the most "efficient" pattern, bit it is thorough. And it's convenient once you have a highway to run from one side of the map to the other. I also have used it to flood hell, which was funny.