Simple right click casting.
Double right-click to cast while a fishing pole is equipped. Options included to enhance the various telltale signs that you've caught a fish (upping the volume, enhance the pool "sparklies") and automatically applying a lure to your pole.
Double click casting can result in lost catches in rare situations where the loot window and casting overlap. I haven't figured out if this is due to a slow computer or a slow connection, nor have I found a way around it yet.
Updated for WoD.