This is a perennial problem for most homemakers--scum on tubs that won't come clean.  

Although this can be a tough, frustrating problem, there are solutions that can keep it under control.  First and foremost, the key is to rinse dirt residue from your body off the tub every time you bathe or shower.  As your mother probably told you, "an ounce of prevention is worth a pound of cure." Besides that, this post will present some practical other steps you can take to make sure that your tub stays fresh and clean.

Steps to Take

Indeed, there are things you can do to keep your tub clean.  The solution can be found in things you may have never thought of as cleaning supplies.  For instance, as Bob Vila suggests, while you can use fresh, clean, warm water as a surface cleaning agent, wiping it down with a squeegee or microfiber rag, you can also employ a cooking ingredient such as white vinegar, mixing it with warm water.  Place this mixture in one of those spray bottles.  Spray it on, wiping it down with a squeegee or a sponge.  
I would not suggest a squeegee rather than a sponge because if you use the latter, remember that sponges hold germs.  That could add more trouble because germs can cause skin diseases you may not want to experience. 
If you're one of those people who tend to put off cleaning your bathroom for weeks at a time, remember that you may have to use agents that will provide a deeper clean, for hard water stains can easily form.  Baking soda comes in handy, according to Vila.  Sprinkle it on in conjunction with the vinegar and water solution.  Scrub it with a soft cloth.  Do this until you've made it into a paste.  Then let your newly-formed paste sit for 15 minutes. 

Hard Water Stains

As  Vila further states, the hard water treatment plan includes dipping paper towels in white vinegar.  Do not dilute the vinegar.  It should be used full-strength.  Place soaked towels on the stains.  Take a 2-hour break while allowing the solution to work.  Then return and scrub the stains right off.  When scrubbing use the baking soda and vinegar paste. Then rinse clean.

Other Tips

According to Readers Digest, you can also use cream of tartar for a bathtub cleaning.  Fill up a small cup or dish with cream of tartar.  Then add hydrogen peroxide.  Stir it steadily until thick paste forms.  Then apply to the troublesome stain. Then let it dry. 
Did you know that common kitchen table salt also works?  You can also use another common household product:  shampoo, which contains soap.  These are easy ways to clean up your bathroom tub.

Conclusion of the Matter

All this being said, remember some safety rules when cleaning your bathroom.  Do not use anything with chemicals in it, such as ammonia, or Clorox Cleanup, which contains bleach--without ventilating your house.  Open some windows, and allow the air to kill the odor.  If you breathe in the fumes, or if the gasses come into contact with your skin, you could become very sick.  In some cases, you could even die. 
Short of that, if you have breathing problems--like I do (I have bronchitis)--it is especially important that you ventilate the area really well before leaving it.  
Above all, the most important item is maintenance.  After you've been in the tub--rinse your personal residue away with warm water and a fiber cloth.  Do it every time.  Even given that you follow these tips, you will still need to deep clean your tub, but if you rinse all the time and not let dirt, hair, grime, scum, and other such problems sit--when it's time for deep cleaning, the process will be easier.