The easiest and best method for removing soap scum is by using Easy Off Fume Free Max in the blue can. It's so incredibly easy and leaves your shower sparkling clean! Since it's a foam, you can spray it on the walls and it stays there. Just spray all over the walls and floor and then leave it on there for a couple of hours. Come back and and use a soft-bristle brush to get all of your soap scum away. No scrubbing involved. It doesn't take away the lime (unless it's embedded in the soap scum), so you may have to follow up with some Lime Away or KaBoom! It's safe for fiberglass showers and floors. I haven't used it on actual tile. It may eat at the grout.
After cleaning, you can lightly rub some car wax onto the walls (but not the floor!) which acts as a water repellant and prevents scum build-up. Be careful not to get wax on the shower floor because it will make the floor extremely slippery.