Pop the onion in the freezer for a few mins prior to chopping
Use as sharp a knife as possible, preferably twice as long as the onion
Work in a well ventilated area
Edit: I found a website that might help alot
Simply keep your onions in the fridge along with other veggies and you will never cry when chopping them - it's that simple!
You can freeze an onion, just prepare like you were going to use it. Pan fry it before cooling and putting into an airtight container and freeze. This stops the onion going mushy when defrosted. Use as you would any other onion.
If you use a sharp knife, there shouldn't be any tears. An onion makes you cry because acid is being released from the onion. Using a dull knife crushes the onion rather than cutting it, releasing far more of this acid into the air.
Rinsing the onion will prevent it from browning.
Try holding a wooden spoon or a piece of bread between your teeth while cutting. This significantly reduces the irritation to the eyes.
Keep your mouth open and breathe through it while cutting onions to reduce the amount of tears.
Burn a candle on the counter near where you're cutting the onion. It'll burn off the fumes that cause eye irritation. A scented candle also smells better than raw, chopped onion!
You can also avoid tears simply by chewing a piece of gum while you are cutting the onion.
Try holding a wooden match in your teeth (unlit of course); it will absorb the odor.