Cut it in half along the equator, hold each half over a bowl and hit the outside hard with something like a rolling pin. If it is ripe, the seeds will fall out! Quicker, less messy and easier than the pin method. Thank Nigella for this, not me!
A friend of mine from Pakistan, seeing me wrestle with a pomegranate and a fork, said British people ate pomegranates in a wimpish way; he said the thing to do is just to smash it against a wall and get stuck into eating each half - quickly!