How do I know when to harvest my onions?

I think that when the tops turn brown and look like they're done being green is what I have heard, so I agree w/ gardengirl...

We harvested ours after the green tops had fallen flat for about a week. Then we laid them out of the sun in a dry area with plenty of circulation to cure for another week. At that point, we cut the tops off and now have the onion in the pantry, ready to use! A couple of pics here: http://goo.gl/p3tv

I wait until the tops flop over and leave them in the ground for a bit longer and then pull them out. I have read to leave them for about 10 days, but I was impatient this year! Then I set them on my patio table to dry for a day or so. I just pulled most of mine out yesterday morning. I brought them inside tonight before a big rain and cut off the tops, leaving about an inch and will let them dry out on the counter from here. Mine are pretty small in size this year and I won't be storing them for long - we will eat them quickly! Onions are fun to grow - I hope you enjoy yours!