1. Wear goggles; well this one is a bit silly, but it does actually work. Perhaps try this one if no one else is around!
2. Freeze the onion for 15 minutes before chopping; this works by inhibiting the release of the chemicals that cause eye irritation. Remember not to leave the onion in the freezer for too long or it will be impossible to chop if frozen solid
3. Chop the onions outside in the breeze, by an open widow with a breeze or by a vent or fan; this is my favorite and most simple method. The breeze or air flow works to carry away the eye irritating chemicals that are released from the onion as you chop.