Spend a LOT of time in a parking lot practicing slow speed maneuvering where you are making turns at full lock on the steering and not putting your feet down. On most normal side streets you should be comfortable making U turns with out even a thought of putting your feet down. This is all before even your wife gets on the back. Then do it all again with the wife.
Before your first ride with a passenger, go to your local home improvement store and buy 2 big bags of play-box sand and strap that to the back seat. It will get you used to the extra top heavy feel, and sand don't wiggle, and if you do go down sand is cheap.
Cover the crash bars with some old garden hose and have the wife wiggle around while stopped. You should also know how to pick the bike up if / when it does fall. Be sure to let us know so we can explain the dropped bike donation.
Get of copy of the DVD "Ride Like a Pro", watch it, do it, watch it again. Repeat.
Even better would be to take an advanced rider course.
I had a situation where while at an event another parent asked if I would take their child on my bike since I was alone, and they had more kids than bike seats. These children were all VERY experienced bike passengers and loved to ride. I had to decline since it had been about 30 years since I had a passenger with me, and we would be riding roads unfamiliar to me and more technical than what I am used to.