Your snorkel should be no longer than 18" and have a large bore. The snorkel should be flexible and fit your bite.
Try to get one that has a replaceable mouthpiece because as time passes, people have a tendency to chew through them on longer snorkeling tours - not out of fear, but from excitement.
Also, there are snorkels that have one-way purge valves. Water can drain out of the bottom, eliminating blast clearing.