Many years ago my husband & I were working on an activities committee for our church & we were in charge of the annual fourth of July breakfast. Also on the committee were a couple of retired women who had been school cooks. One of them taught us this handy way to cut melons. It is so quick & efficient & makes it a snap.
First, cut the watermelon in half (not lengthwise).
Place the heart of the watermelon down on the cutting board & slice off the very top slice.
Then, starting from the first cut, proceed to cut down, cutting off strips as you go.
After you have removed the rind, slice the watermelon (horizontally) into several rounds.
Then, slice several times vertically until your cubes are the appropriate size.
Place the serving bowl upside down on the cutting board, & rapidly invert.
Source Recipe: http://triedandtruefavoriterecipes.blogspot.com