There is a great wiki with some number crunching.
I use this in single player:
▒▒██▒▒__▒▒__ ██ blocks you should mine
▒▒██▒▒__▒▒__ ▒▒ blocks you can see
__▒▒__▒▒██▒▒ __ blocks you can't see if you don't mine ▒▒ blocks
__▒▒__▒▒██▒▒ Repeat this pattern as many times as needed
Normally, I have a 2x1 trunk, and branches like a standard branch mine, but instead of just going outwards from the trunk, I go up one level and out, skip two blocks, then down one level and out. I leave two blocks between branches. And I leave two blocks between floors, and line up the branches the same between floors.
There are some areas left unexplored by this pattern, but they are thin, and it is unlikely that a diamond vein with spawn entirely within that narrow region. (About 3% will). The larger number of blocks uncovered are far more likely to contain diamonds.
If your high density branch mine gets you 100% diamonds, the low density branch mine would give you 212.6% ores in the same time, minus 3% from the earlier probability worked out before.
Also, you ought to build it so that the floor of the lowest tunnel is on level 11, to avoid falling in lava. If you find that there are few cave systems near your mine, feel free to dig lower, but caves tend to cluster, and caves level 10 and below are filled with lava.
I dig my main tunnel on both sides of the area I want to cover, then dig back and forth in a zigzag patten, which means I'm always digging, never walking.
I'm not sure if this is true anymore, but it used to be that larger deposits were formed by the word generator in multiplayer, so you were less likely to miss diamond deposits when using a wider mining pattern. Thus, this pattern
▒▒██▒▒____▒▒____ ██ blocks you should mine
▒▒██▒▒____▒▒____ ▒▒ blocks you can see
__▒▒____▒▒██▒▒__ __ blocks you can't see if you don't mine ▒▒ blocks
__▒▒____▒▒██▒▒__ Repeat this pattern as many times as needed
is more efficient.
As for transit? I use a long two wide straight staircase that goes all the way to bedrock. Minecarts work well for getting me deep into the mine.