Even better--hold the half-pomegranate under the water and pull it apart with your hands (thumbs on top, pull toward you)--no red juice flying around, and David is right about the white pith floating and the red arils sinking (for the most part)

Cut in half crosswise. Hold hemisphere seed-side down over a large bowl filled with water. Beat on the hemisphere with a wooden spoon to force the seeds into the water. Seeds will generally sink; detritus will float and can be skimmed off. Drain into strainer or colander to collect seeds. Warning: do this in an area that will not get stained by flying juice (I just lay newspaper on the counter or do it in the sink) and don't wear a white shirt!