Remove the shower head if possible
Fill a bowl or bag with white vinegar
Submerge the shower head in the vinegar
Soak for 30 minutes to several hours
Scrub the nozzles with an old toothbrush
Use a toothpick to clear clogged holes
Rinse the shower head with warm water
Reattach the shower head
Run hot water through it for a few minutes
Wipe the exterior dry with a clean cloth