Works every time

Or keep a mouthful of water while you're cutting them.

Cut under running water, put a damp cloth over your nose-mouth area, or don't breathe right in front of them while you're cutting. The part that makes you cry works because you're inhaling it.

or chewing gum

Or just store onions in the frigde. Bring it out just when you're ready to chop them. And don't wear your glasses if you can help it.