This is something that I have done for as long as I can remember. I cannot remember where I learned it from. It only works on smaller fish, so I havent used it in a couple of years with my cichlids, but it works great nevertheless.
Take a plastic waterbottle or two liter bottle. Obviously, make sure the bottle is clean and tank-safe. First, remove the cap. Cut off the top portion a couple inches below where the neck turns into the straight portion of the bottle. Now you have two pieces, the one containing the neck and a elongated tube with the base of the bottle. Next, take the neck piece and place it inside the other piece with the narrowest portion facing the base of the bottle.
Warning: terrible attempt at visual aid.
......1.........[_ <-------Fish go this way
This system on the basis that it is very easy for the fish to swim into the area 1, yet difficult for them to come out. Once this is setup, sink it into a corner of your tank, and corrall the fish in that general direction. Once you have the fish in area 1, simply pull the bottle out of the tank, remove the neck portion and either net the fish out of area one into the new tank or simply place the bottle with neck removed into the other tank so the fish can freely swim out.