Practise (and get used to removing and replacing the decor). Sorry, but if we're talking rock-dwelling fish, no magic solutions, and I've tried a number of the ones that are supposed to work. "Nab'em while they're sleeping" - well they sleep in nooks and crannies where the net won't go, and don't take that long to wake up when they sense you're after them, ime. "Pop bottle trap" only seems to catch the fish I don't want to catch. The ones you want to catch always seem to be more weary because in their condition (holding, sick, injured) they feel more vulnerable. I supposed there may be more elaborate traps to try, but these are very wiley fish!
Actually here're some things I do to reduce the frustration involved. Try to time it with a water change (catching fish in a half full tank is much easier). Remove the decor from one end of the tank to create an open area. Use two nets, one in each hand, and try to chase the fish into one net with the other, or failing that use both to chase it out of the rockwork into the open end, then use the two nets to catch it. I use two different size nets and use the smaller one to chase the fish into the larger one.
If you anticipate needing to do this often you can rig up something re-usable the right size to divide the tank with, at least half-way up. A leftover piece of eggcrate can work, for instance. Put it in after you've lowered the water level, then you only have half the tank to catch the fish in.
Good luck with it.
