You could also try to hold the food directly infront of you so it can see you while focusing on the food. It helps to not divide their attention too much.
Well it's not as simple as teaching a dog to roll over or sit but just take a cricket hold it between your fingers in front of him/her and if they so desire they will eat it. If it doesn't work the first time just keep trying and trying until eventually the Cham eats from your hand.