I have always opened my pomegranates by submerging them in a bowl of water and deseeding them. Preventing the juice from splashing everywhere. But I just saw a life changing video of Martha Stewart doing it a much better way. She scores the peel with a knife. Opens up the pomegranate. Then holds it in her hand, seeds facing down. Then hits the peel with a good heavy wooden spoon. Which then knocks all the seeds out of the pomegranate.
Check it out here. Because my explanation is probably confusing.