1 cup (250ml) vinegar
Over time, mineral deposits can block the small nozzles of your showerhead and affect water pressure, but a 12-hour soak in vinegar can do wonders to get rid of them. Soak a clean washcloth in 1 cup (250ml) vinegar and wrap it around the showerhead. Alternately, you can give the fixture a direct soaking by filling a 1-gallon plastic bag with vinegar and affixing it to the fixture with some duct tape (to hold it in place). Let sit overnight and then remove. Your morning shower should be noticeably improved.
CLR is a commercial option which is also earth friendlier than many products on the market