We live in a HARD water area too, I use a product like Viakal (can't remember the name of the actual product) from Lidl which I use about once a month and give the shower a really good clean, I use this on one of the non stick kitchen scourer pads, this gets rid of all soap scum and water marks, then during normal cleaning, I use the limescale wipes which I get from B & Q and everyone is trained to squeegee everytime they shower, this keep my shower doors looking good. Even my sister when visiting recently asked how I keep the glass so clean.