How do I drag an item behind my vehicle in Mario Kart Wii?
You just hold down whichever button is normally pressed for shooting the item.
As soon as you release the button the item will then fire, and as far as I know you cannot stop holding the item behind you without firing.
This works with single shells, single bananas and possibly the fake power-up boxes.
It seems like when using the Wheel, it is by holding left or right on D-pad.