This blog post has some excellent advice on grip and hold for improvised weapons. Thrusting is not recommended unless you are confident of strong wrists and use of small objects as fistloads seems to be the central notion here.
From the article: One thing I would firstly like to mention is the way a weapon is used. There are three main movements in which to strike with an Improvised Weapon: You can swing a weapon much like a bat or stick, You can thrust a weapon like a knife or You can stab much like a knife wielding maniac but with a blunt instrument. This movement is using a hammer fist type of strike. It is really up to the individual and their preferences as to how any Improvised Weapon is used.