A build up of mineral deposits on your shower head can slow the spray down to a trickle. A couple of easy steps can solve your problem.
Heat up some vinegar and pour it into a plastic sandwich type bag. Slip the vinegar bag over your shower head making sure to cover the entire spraying surface and tape it on securely. If possible let the shower head soak over night. In the morning take a small bristled brush and brush off any stubborn deposits on the shower head. Run hot water through it to clear it out.
If you still have some holes clogged you can take a tooth pick or a needle and poke the remaining deposits free. Note – If you have a gold or brass finished shower head they are usually coated with a plastic protective film and should only be soaked for less than a hour at a time to prevent any damage.