Once you dice garlic this way, you never go back. It's much faster and more precise than doing the mad dance with the knife I used to do. It seriously takes less than a third of the time, and the diced pieces come out more uniform and organized than my old method. How do you do it?
With a sharp knife (I use my giant kitchen knife), slice the garlic, not cutting all the way through to the root end. I find it easier to hold the garlic on its side between my left thumb and forefinger, bringing the knife down between them to slice. Try to start with the "round" side (there is usually at least one flat surface you can lay the garlic down on).
Then, place the garlic down on one of its flat side. Make slices the other way, not going all the way through the root end, kind of like a checkerboard. I find this easiest while squeezing together the already sliced parts so that they don't fray when you slice the other way.
Then, dice, beginning from the end:
Voila! Beautiful pieces of diced garlic!
If you're picky or if you're not in a rush, you can cut the garlic in half first, then remove the green sprouty part by digging it out with your fingers or a knife. Proceed as before, using the sliced edge as the flat side.
This technique also works well with onion, but cut them in half and remove the rooty ends first. With onions, you can also eliminate step 1. Just go straight to making the cross-ways slices, since onions already have natural layers that act as your hatch-ways slices.
May your chopping and dicing be forever faster and more precise, henceforth!