Is the diverter valve leaking? If you have a three valve shower, it's the handle in the center. If you have a two valve or single-valve (delta ball valve) it's the lever on the top of the spigot.
If not, take the shower head off and bang it out and back-flush it by running high pressure water into it from the discharge side. You will have to use your hands to cup a seal or take it outside and press a garden hose up to it and blast it. This should hopefully remove any loose scale that has accumulated.
If there is a filter screen on the inlet of the showerhead, that could be the problem too.
While you have the showerhead off, run water through the shower pipe - make sure scale isn't comming out and that you have good flow.
Lastly, the vinegar sugguestion presented above might work, although for it to plug up that quickly, it'w probably a loose/floating obstruction/restriction rather than a buildup of lime/calcium/scale. One thing about the vinegar soak to be aware of is that it might damage the chrome finish on the showerhead. Alternately, you could squirt some CLR (Calcium, Lime, Rust) remover into the showerhead from the inlet side, then let it sit and then run water through it.