There isn't any certain method, but:. . a) make sure you have ventilation, the problem isn't juice squirting in your eyes but the vaporised gas, so open a window. . . b) Use the sharpest knife you have - the more cells of the onions skin you break the more fumes will be released and the worse it will be. . . c) try to keep your face back a bit, not directly over the chopping board.. . Your eyes will still hurt, but maybe a bit less. They used to say stick a metal spoon in your mouth upside down, I think the idea is to keep your mouth closed and breath through your nose (which will help); as for the spoon it only gives you tooth ache as you grit your teeth through the tears.