I have been riding for just over a year now and waited 5k miles before allowing my girl on the back (then shortly after having to buy her a bike because she got hooked). A few things that might help your troubles:

When braking it should be much smoother than alone and take it slow. For her sliding forward, she should brace herself with her hands against the gas tank in front of you. Doing that is much much easier on you than having another 100 pounds bracing against your back and then going into your arms.

As for shifting with a passenger, I have found it easier to do a slower shifting, clutching in and out slower/smoother than usual. I'm assuming the shifting trouble comes from the extra weight being shifted forward and back when disengaging the power then re-applying it.

I hope my noob ass tips help you out :P