You can only try to shorten the distance to the attacker as soon and quick as possible as the chair, baseball bat or any other object he is potentially throwing has the strongest impact on the end of the motion or object.

You could use another object, back pack as a shield to protect you, if available.

If there are multiple attackers, try to get one of the attackers between you and the threat (having him as shield) and as close as possible to him. (Hoping that the original attacker is a good friend of the guy and doesn't want to hit his friend ;-))