One way to level up, is to fish constantly in a major city pool (not in Dalaran). You avoid get-aways and you can level faster with out wasted casts. But this method just obtains fish that are pretty much vendor trash.

Another way is to go around to a zone that fits your level (without get-aways) and fish there. These can get you a wide variety of fish, which can sell at a decent price. However, this is very time consuming and spend a lot of planning on which zone to go next.

The method I choose to use, is fishing by Wailing Caverns. You can fish up Deviate Fish there. When cooked as Savory Deviate Delight, these fish sell for on average (across servers) 1.5g each. Not a bad chunk of change for leveling fishing. The reason why I chose this method (instead of flying around to zones) is they hold their value and sell rather quickly. Also It is nice to go to one place. Set up a TV show or something to watch, and fish for a little while without planning your next zone. Another reason why I choose this method is that the mobs are very low level, so it makes it a good farming spot for almost any level.