You can protect yourself when a blue shell is about to hit you.
But it take patience.
Once you got a boo/ghost as your item, get into first place.
(NOTE: The boo is what you need to protect yourself, so don't use while you
are trying to get into first, so this may be a little difficult)
Once you got into first, wait for a blue shell to approach you.
Once a blue shell is on the map coming towards you, quickly press the X button to release
you ghost AS SOON AS POSSIBLE!
While your ghost is away looking for an item to steal, the blue shell will hit you.
Once the blue shell hit you, you won't go flying up in the air, you will stay perfectly still driving.
The ghost has protected you, keeping you in a force field, also if you are lucky the ghost might
have stolen a useful item from somebody and have given it to you.