Every few months, despite my frequently emptying the hair-trap, my shower drain gradually becomes unable to clear the waste water fast enough.
I imagine there is a slow build-up of soap scum, escaped hairs or whatever in the almost horizontal under-floor waste pipe: a partial blockage.
There's plenty of choice as a remedy: hydrochloric acid (also excellent for removing underwater limescale in toilet bowls); sulphuric acid preparations (such as One Shot); sodium hydroxide (aka caustic soda, alkaline) preparations. In concentrated form, these tend to be highly corrosive, brutally effective, and fairly expensive. Handling is not for the faint-hearted.
Keep It Handy liquid drain cleaner contains (among other things) a relatively low 7-8% sodium hydroxide, though it is still classed as corrosive and an irritant. It sinks through standing water, thus remaining undiluted, to attack the blockage. I generally leave half a bottle to work overnight (although you don't have to wait so long) and hose it through next morning with hot water.
I find it to be an easy-to-handle, very effective and (at this price, £8 for 8x 450ml bottles) economical remedy, which should last me several years.
PS (2 Apr 2012): Just used some down my kitchen sink, which was taking all of 40 seconds to drain from full. It now takes only 20 seconds.