I use limescale remover and then everynow and again vinegar, but if you buy one of those scrimmer things (like a windscreen wiper) that the window cleaners use and use it every time you shower it will need less of this treatment.
Spray with 'Flash' glass cleaner, rub with non-scratch scrubber,then use a scrimmer.
Always use 'Daily' shower clean spray after ever shower which you just spray & leave. This makes it easier to clean.
Try Mr Muscle's Shower Shine. I used the bathroom at my parents house for the first time in years and was horrified by the state of their shower screen. It took me a few days, several sponge scourers and a great deal of elbow grease to get it clean. Now it is clean I've told them to spray it every time they use it. Touch wood, it's stayed clean.