As difficult as it is, there are a few techniques to being good at diamond mining. Diamond is found between layer 10 and 16 (press [F3] to see what level you are at), which greatly reduces the volume you have to search for it.
There are a few designs for efficient ways to strip mine for diamond.
(1) Complete coverage design. Make a 2x1 tunnel, spaced every four meters (blocks), so that there are 3 blocks wide in between each tunnel. Do the layer on top, but offset it by two, so that the tunnels are above the center block of those between the layer below
(2) The “quick-and-easy,” where you go to level 11 (so you are walking on level 10), and dig a long tunnel, stopping every 3 blocks to make a branch at head height that is 1x1. You will not be assured that you will not miss diamond, but it is very quick. Also, you are walking on level 10 because this is the level lava usually spawns at, so be careful not to walk into a pool of lava when excavating forward.