Mr. Clean Magic Erasers
I keep a dish scrubber in a tall cup on the side of the bathtub. The scrubber holds a mixture of dish soap and vinegar. After anyone uses the bathtub I scrub the soap scum off with the dish scrubber and rinse. I never have rings with this method because it is done after each use. For deep clean scrubbing I will use soap scrub. Keep one of these next to the bathtub:
Here's my favorite:
Hello- this Hometalk, Bathroom bathtub-How to Clean the Tub Without Breaking Your Back DIY link might be helpful.
For soap scum, you can use inexpensive hair shampoo and water; the clarifying type is the best. If you need something more robust, fill the tub with hot water to the overflow drain level, and a quarter cup of dishwasher detergent. Swish to mix, and use a sponge to dampen the sides above the water line. Let sit for an hour, give it another quick sponging and then drain and rinse/wipe down with clean water.