If chopping onions makes you gush rivers of burning tears, it's time for a few changes to your approach.
First, use the sharpest knife you have. Chopping onions ruptures the veggie's cells, releasing noxious compounds into the air. The sharper your blade, the fewer ruptured cells. If all your knifes are dull, time to sharpen them. Check out this knife sharpening video for simple DIY sharpening tips, or take them to your local specialty market, which may sharpen them for you for a small fee. Another benefit: Sharp knives make all your kitchen tasks a little safer, since they slip off food less and you tend to use more care when handling them.
Next, position yourself near or below a powerful hood vent. Set the ceiling fan to high and bring a standing fan into the kitchen and let 'er rip as well. The idea here is to increase air circulation and keep the onion fumes from traveling straight from the knife to your nose.
Solid technique also helps. If you know the classic French method for dicing onions, great. There are also effective alternatives. But the important thing is to have some kind of efficient system for cutting onions rather than hacking haphazardly at them.
Finally, a tip that may sound ridiculous, because it is. But it's also ridiculously effective, and a last-resort technique I've put into play more than once. A diving mask and a snorkel. Go ahead, laugh it up. But when I have to dice a dozen enormous yellow onions for a party-size batch of something or other, I'm reaching for the no-tear gear.