Patience. I've had success getting damsels with a little bit of food, in a jar. Don't bring the net into the tank at all, just slowly submerge the jar, put a bit of sinking food in, and then stand there and wait for the damsel to come check it out. You can't move the jar towards them--must be patient! When the fish swims in, slide your hand over the opening and remove.
It's worked on a blue devil 2-3 times, as well as the bicolor angel and the engineer goby. Works very well on shrimps and crabs, if you put the jar on the bottom of the tank.