What are you driving, a Dodge Magnum? :-)
I usually have a pair of boots and my wife's skis over one shoulder, and my skis over the other. My wife carries her boots and both sets of poles. If it's just me carrying just my stuff seems light.
You definately don't want sharp skis flying around inside the car. Those SGs could take your head off :. Keep 'em below the seatback and don't roll over. The roof box would be safer, but my gas milage is already bad enough.