Re: I only know of chemicals
cilit bang is great, works wonders on shower doors, taps etc
But do be warned - do NOT leave it on longer than the recommended time of 1 min, I heard of a person on the radio who used it and it took all the enamel off her bath. Its also not suitable for all surfaces so read instructions.
SuperValu own brand do a limescale remover in a blue bottle, it is great also, wonders wonders in the loo but they do say not to use it with septic tanks. I used it on chrome taps and it discoloured them.
Other than that get yourself a squeegee (sp?) and an extra 30 seconds in the shower and clean the shower door after every shower. You'll only have to clean once in blue moon.