If it is not a big tank then I advocate the "surprise them in the middle of the night by turning on the lights" method. All the fish in your tank will be sluggish for about 30 minutes or so. I try to do this after a water change.
(1) On water change day, siphon some old water into an old bucket.
(2) That night, catch the fish and transfer it to the bucket.
(3) Ship/return fish to LFS/wherever the following day.
This way, you don't have to wonder where to store the fish until morning.
The flash light method never worked for me. Only succeeded in giving me frustrations: trying to point with one hand and catch the fish in the other. Some days, you just wanna be an octopus. ::squirts::
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If it is a large tank, then you'd have more success by rigging a fish trap. I can't find a link right now but I recall cutting the top half of a soda bottle and invert it so that the neck and opening are facing the bottle's bottom. Put some food in there. The theory is that the fish will swim into the bottle to get the food but is too stupid to find its way back out. Those SAEs are clever little buggers though. :toimonst: