From Lifehacker: Tie a Bag of Vinegar Around Your Shower Head for Easy, No-Effort Cleaning [Clever Uses]

Your shower head is prone to buildup of all sorts of nasty stuff, and it can be a pain to clean if you do it the hard way. As Redditor IT_Serpent points out, you can cure your shower head of its grimy blockage with a bag of vinegar. All you have to do is fill the bag, place it around the shower head so it's submerged in the liquid, and tie it to the neck with a twist tie (or anything else, really). Leave it there overnight and the cleaning will be handled for you.
The picture above shows how this works. It's a pretty simple trick to doing an otherwise frustrating job.