What I prefer to do is this...

1.  Insert a long locking blade knife into the head and push the knife along the line. This will kill the fish quickly, so you can clean it right away. 
 
2.  Cut in an oval shape along this line, to get the most meat. Cut in the same line almost diagonally along the fish to not have to mess with the organs. If it is a male, this is easy to do, just cut behind the claspers. When cutting through the spine be careful, the fish will jump even though it is dead, and the flesh will move as well, it is pure muscle in the tail.
 
3.  You can make fillets, but I like to make medium sized chunks, and cut out any red or dark meat. The chunks make excellent fish and chips, or can be grilled.

 

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Courtesy of Adam Cassidy, A.k.a. "Baitfish"