Make sure your cutting board doesn't stain with any other flavors because it will absorb it.
If your watermelon has seeds, make sure to remove them.
If you are sensitive to bitterness, a lime will produce that even the little white pith in between the flesh. Proceed with caution because if it's bitter that is your lemonade and not mine.
When you're ready to serve, you don't want to water it down with too much ice.