How to keep from crying while slicing onions?

Cut them next to an open flame. Either a gas stove, a few candles, Bunsen burner, etc. Cuts through the vapors. Also, make sure they are chilled. Both of these methods together = no tears.

Put them in the freezer for a little while before you slice. This freezes the liquid inside and prevents the vapor from forming.

get a small chopper/food processor & let it do the work

If you soak the onion in water before and then cut the onion into a bowl of water, you'll hardly feel the effects.

I buy my onions diced at the store. No more tears.

Have the wife/girlfriend chop them...

I'm sure there have been a lot of suggestions for you to cut them next to a candle.

Take it from someone who has spent a ton of time chopping onions- the only that that i have ever had be useful is if they're frozen (not to the point of being unable to cut, just frozen enough so the onion juice is solid).

Here is the best answer of all. When you are in the kitchen cooking most times the stove is on and it is hot. I put a small fan on the counter and face it towards me while I am cutting onions.

No tears.

That simple.