Depends on whether or not you can take evasive action and what part of your car he is sliding towards. If you can't take evasive action and he is sliding towards the driver's side, then try to hop across to the passenger side where you are less likely to be crushed. If towards the rear of the car, then brace your head against the head restraint to try to prevent whiplash injury. If towards the front and you can't reverse, also brace your head against the head restraint and hope the airbag goes off. If you can get some movement into the car away from the impact before the impact it will reduce the severity of the shock. Be prepared to sacrifice your car's crumple zones rather than yourself.