http://www.similicious.com

You can buy a SIM card from them, they'll mail it to you. When you land, just put it in your phone and you're good to go. It's the simplest, most popular way to do it and I've done it a bunch.

Only one thing I say is it's a little expensive. I've recently just been getting to Vancouver and queuing up a map on the airport wifi that routes me to the closest Rogers. They are much cheaper there. Just make sure you bring a US Debit Card to pay for it.