My dad showed me this really easy way to dice an onion and I now do it this way all the time.
After peeling your onion, slice off a few slices to your desired thickness. I was making pico de gallo, so I didn't need them too thin.
Next, stack 'em up.
Then slice the stack in half.
Move one stack out of the way.
With the stack remaining, start cutting on the edge away from your hand.
As you cut, turn the onion so you always have a pointy side to cut off (more or less).
Voila! Diced onion!