This previous question was more about cleaning the shower head and not specifically unclogging the nozzles. I have a Speakerman Anystream 2000 massaging showerhead whose nozzles are so clogged that only a trickle of water comes out now. There were alot of hard water stains that I cleaned off and tried various things but it is still clogged.
We have hard water and we use a combination of physics and chemistry.
Physics: rub your thumbnail hard across the little holes, and usually the scale will flake away. Some people are also fans of poking toothpicks, safety pins etc into the holes.
Chemistry: get some CLR. Either remove the shower head and set it in a bowl of CLR, or do what they show on TV and hold the bowl up to the in-place shower head for a while. (If I were to do that I would wear goggles just in case.) You can reuse the CLR to clear scale off other nearby things after doing the shower head.
Take a baggie, fill it with Vinegar and tie it around the shower head and let it soak over night. Remove the baggie and flush with water. The vinegar, being acid dissolves the lime which is a base. Alternatively, if it's easy to unscrew the shower head, remove it and soak it in a small bowl of Vinegar. Vinegar is a lot less expensive than commercial cleaners and does a remarkably good job. Vinegar is a great to have around the house: