Ever since I've defeated the Ender Dragon I've been preparing to go against the Wither Boss. What is the easiest way to defeat him?
If you are looking for a farming type of setup, you can create a chamber in the top of the Nether of 3 blocks tall with the roof being the bed rock layer. In the floor place a piston pointing up. Build the Wither on the piston and as he is spawning, activate the piston to push his head into the bed rock block. This keeps it from moving around and taking shots at you.
Here is a video that demos it with anotherof SethBling putting it to use.
Here is a example of fighting in a tight enclosed space that isn't totally safe. Basically the space is only large enough to spawn it in and for you to stand and watch.
Spawn the Wither in the End.
1. The enviorment damages are tiny.
2. The Wither mostly attacks Endermen.
3. The Endermen get angry and attack the Wither, softening it.
Just make sure that you are properly armoured, that you have a good sword, and an Enchanted Golden Apple, and eventually, bows and arrows.
Then, skirmish at it and after it falls below 50% health, eat the Apple and attack it with the sword.
Also, make sure that the Wither doesn't fall into the Void.
Trap the Wither into the Ender Dragon's nest using a piston.
The existing answer whereby using a piston to trap a Wither in the roof of the Nether can work. However, I have had problems finding a flat bedrock section. On occasion the Wither can break free through gaps in the bedrock. This is without mentioning the hazards of mining upwards without getting killed by hidden lava pools or ghasts knocking you off your method of climbing.
I found that if you go into the End and mine a small area under the dragon's bedrock nest (make it 3 blocks tall), you can use the piston trick to trap the Wither without it getting free.
As soon as the Wither spawns activate the piston (e.g. a red stone torch), and either back track or hold block to counter the first explosion. Proceed to kill the Wither with any means you wish. He will be stuck into the bedrock ceiling.
Of course, this assumes you have found the End portal (and is a lot easier after defeating the dragon).