Easy Reel-In:

Once the fish bites and starts swimming quickly to the left, tap "X" once. When the fish slows down, reel it in. It should not struggle. (This does not work on the "Lake" level.)

Quick Reel-In:

In the stream creek level fish as you normally would, and when you see the fish about ready to bite the bait, look for the bubbles that come out of the fish's mouth, when this happens press X and O simultaneously and the fish will automatically reel in without snapping the line.