回複：it's easy but you need some luck

You don't have to go to Canada that late, i.e. 1 or 2 days prior to the expiration date of your I-94, which is actually very suspicious to the US immigration officer. It's easy. Catch a flight to Canada any day but remember to leave the I-94 with the gate agent before boarding. On the way back, show your valid B-2 visa to the US immigration officer in Canada (this is done in the airport in Canada). Here, you need some luck. It's not guaranteed that you will get another six months. I've done this several times and managed to get six months most of the times. I don't recommend driving because when you go into Canada, the Canadian immigration officer doesn't want to keep the I-94. And remember to get Canadian visitor visa before going. Good luck.