For netting, I remove large obstructions, lower water level if possible, and once I confine the target fish to a corner, I slide in a tank divider to make the escape zone smaller.
For catching smallish fish I take a 16 or 20-ounce soda bottle, cut off the top right below the neck, take the piece I cut off and put it back in, inverted, so that the neck of the bottle is now inside it. I put some delicious food in there, and the fish swim in the small opening but can't figure out how to get out. You catch unwanted fish sometimes but usually it is pretty fail-safe.
Great thread, Menagerie!