Step 1: The Haves
* A zipper type bag (or a plastic bag from a retail store, but obvious it must not have any holes)
* A rubber band (you could also use any kind of tape you have laying around)
* Distilled Vinegar (if you have access to a higher acidity vinegar (20% acidity) this would work even better, but regular strength (5%) works perfectly well . . . and apparently this picture indicates that Kroger's brand is made of grapes, bananas, tomatoes, and practically every fruit or vegetable known to man. I call bulls**t on that.)
Step 2: The Could Haves
If you have a really funky shower head then it might need a little bit of scrubbing. You can do this with one of the myriad unused toothbrushes you probably have lying around from previous dental visits (don't they give out the crappiest toothbrushes?). A scouring pad would work as well.
Step 3: The Doing
* Face your adversary and fill the zipper bag about 1/4 to 1/3 full of vinegar.
* Tilt the shower head downward and cover it with the bag, making sure to completely submerge the head in vinegar. Work out any bubbles that will hinder the vinegar contacting the shower head.
* Using the rubber band, secure the bag to the shower head.
* Allow the vinegar to sit in contact with the shower head over night.
* The next day remove the bag. You can save the vinegar and use it for another shower head. Also you could use the left over vinegar to clean faucets, drain and overflow covers, or whatever has a scale build up on it. I wouldn't suggest drinking it.
Step 4: The Finale
For me this didn't require further scrubbing. There were a few little bits of scale, but I can live with that. It'll give me something to do in a few weeks.
Step 5: Improvising
* You could do this with any kind of cleaner such as Limeaway, CLRClear, or any number of others. Vinegar just happens to be super cheap and you don't have to worry about it splashing in your face and doing permanent damage.
* If you have a horrendously scaly shower head you might consider removing it and putting it in a pot of vinegar on your stove top. You'd want to take care when applying heat not to let it get out of hand so you avoid melting or compromising sealing components of your fixture.
* If you have what some mistakenly call "pink slime" building up on your shower head you can add hydrogen peroxide or bleach to the bag. "Pink slime" is actually a bacteria called Serratia marsescens. It's typically harmless, but in immune compromised people it might cause pneumonia, wound infections, or most commonly urinary tract infections. Just dowse it with bleach or peroxide and you'll be set to jet.