wear goggles.

Using a sharp knife is the best way to avoid tears. There are many other techniques that some people swear by. Running the onion under water hurts the flavor but does take away some of the sting. Wearing goggles works to an extent, but those are made for keeping out liquids (not onion fumes), so they're not effective indefinitely. Some say burning a candle while chopping the onion will help burn up the fumes--and it does--but a small flame can only handle so much. Basically, when you chop an onion you break the onion's cells and that release amino acid sulfoxides (which form sulfenic acids). Once those are out in the open they can produce propanethiol S-oxide, which is what your eyes don't like. Heat renders this enzyme inactive, which is why you don't have the same issue once the onion has been cooked. Just as heat helps, so does cold temperatures. While it won't necessarily solve the problem, refrigerating your onion before chopping it will slow down the production of propanethiol S-oxide and give you a little more time before the fumes waft upwards and get you teary-eyed. It also depends on how you chop the onion, as the cut parts are releasing the fumes. Placing those parts down on the cutting board can help avoid the fumes (see this technique video: http://how2heroes.com/videos/techniques/dice-an-onion). Ultimately, though, if you're as sensitive to onions as I am you're not going to solve the problem with just one of these solutions. Try combining a few and see what works for you.