Hi, I am not exactly an expert at throwing knives but my technique is pretty similar to the throwing of bo shurikens, it works pretty well at throwing light flat topped knives (like the common steak knife) with acceptable range, accuracy, and penetration (6-10 metres on average, I throw at a 2 by 1 foot MDF board).
The basic technique is to put the knife into your hand so that when you hold your palm towards the floor, the blade of the knife is pointing towards the ground. To hold the knife you wrap your thumb over the top of the handle (near where the index finger grip indent would be on most knifes) and you gently hold the blade using your index and ring fingers pressed sorta into your middle finger, kinda hard to explain. The knife is thrown with a downwards whipping motion, letting the blade slide out of your hand (almost stroking it) to ensure it flys nice and straight.
Sorry if that was a little cloudy