Some people use Shower to Shower to clean and keep clean the inside of the showers, just use after each shower. However, we hate using toxins so my husband ordered a Hurricane Turbo Scrubber. It comes with 5 or so brushes and recharging station. It takes all the "hard" out of hard work. You can get them now at Walmart and possibly other retail stores. It is also good for any difficult cleaning such as tires, stove range, ovens, anything. We use our usual non toxic cleanser and it does great.
I clean professionally and I use a soap scum remover such as scrubbing bubbles or lysol power bath. Spray it on, let sit for several minutes, then scrub gently with a little nylon scrubby. Rinse. This shouldn't need to be scrubbed if you keep up on it. Just spray, let sit, rinse.
I mix dish soap and baking soda with some water to make a paste. Smear it on the walls, then scrub with a stiff brush (I got a round Libman pot scrubber at the dollar store just for cleaning the shower). Takes a little elbow grease and some time the first time, but if you keep up with it, future cleanings are much easier and faster.
Cleaning a bathtub or shower with a fabric softener sheet can remove scum. White vinegar is a good remover of residue.
I agree with the vinegar. It cuts soap really well. Just put it in a spray bottle and spray it on, let it sit a minute if necessary and scrub with a non abrasive sponge or scrubby and rinse.