I found this on about.com
Cleaning a showerheadYou can easily remove the hard-water and mineral deposits that build up in a showerhead using warm vinegar in a plastic bag:
*Pour warm vinegar into a small plastic bag.
*Attach the filled bag to the showerhead using a rubber band.
*Soak the showerhead for an hour.
*Remove and discard the vinegar.
*Remove the loosened deposits with a toothbrush.
*Clean the spray holes in the showerhead with a toothpick.
*Run warm water through the showerhead for several minutes.
You can use a similar procedure for cleaning mineral deposits from a bathroom or kitchen faucet head.