Masks, as for sticks and fins, are another very personal thing. Here's a few tips from my personal experiences:

1. Comfort - The most important thing to consider when buying a mask is comfort. You can concentrate on the game if you are not comfortable with you gear. It should sit comfortably on your face and not leak.

2. Vision - You want something that is reasonably low profile that doesn't cut your field of vision down too much. Being such a 3 dimensional sport you want to be able to see as much as possible when you can have players attacking you from above, front, back, sides, etc. Tunnel vision is a common downfall of many masks. You also want something that's not going to distort your vision either.

3. Tempered glass - After seeing several mask being smashed, some at close range (ie 2cm from my own face!), tempered glass is a must.

4. Tight fit - There's nothing worse than having your mask full of water everytime you come into close contact with another player, so make sure your mask can take a few knocks and still stay on your face. Some of the guys are now using twin nylon straps instead of the standard rubber mask strap which they reckon holds their masks pretty firm.

5. Protection - Having something that offers a bit of protection for you face is also a good idea. I've seen a few people with those very close fitting wrap around style masks, and whilst they claim they never get knocked off, I've also seen the damaged from a stray heel when these maks don't offer any cusioning.

Remember that these are only guidlines and each player has to decide what's going to work best for them.