Not sure if this is the right forum for it, but anyway:http://i.imgur.com/QdorsYS.jpg
Recently, I decided I need an efficient farm so looked on the wiki for designs. The most efficient flat design is a 9x9 square with the centre being the water source. Using this method, you get 80 blocks of hydrated farm land for 1 block of water.
This is still a fairly small farm, however, and doesn't get the full potential. I found that a good way of expanding upon this is to have an 18x18 far, which is basically 4 9x9 farms in a square.
This way, you can plant exactly 5 stacks of produce.
To farm it easily, I'd recommend putting glowstone directly on top of the water source. This prevents you from falling in the water and provides a good light source.
However, you'll also need a light source placed around the centre of the farm. As you can see, i've placed glowstone on the corner blocks of all 4 farms, 2 blocks in the air so it doesn't prevent you from replanting.
To harvest, take a bucket of water and place it on top of each glowstone block (not the floating ones in the centre ofc). Replanting can be done fairly efficiently as well.
Planting 5 stacks of potatoes, I can easily get close to 10 stacks in output (after having replanted).