liquid-entrenched smooth surface (LESS) developed by Penn State "When it dries, the first spray grows molecules that look like little hairs, with a diameter of about 1,000,000 times thinner than a human's," said co-developer Jing Wang. The second spray acts as a lubricant.
LESS is already on its way to the market through Spotless Materials

How do you paint a toilet bowl?
Krylon Gloss White Spray Paint (NET WT. 12-oz) in the Spray Paint department at Lowes.com
Rust-Oleum Specialty Gloss White Oil-Based Appliance Epoxy

If tough stains remain, scrub the inside of the emptied bowl with a pumice stone, which will loosen hard-water stains without scratching the porcelain surface.

Pour 1/2 cup of vodka into your toilet bowl each month to keep rings from forming

Pour some toilet cleaner into the tank. After a routine cleaning, add a few ounces of liquid toilet cleaner or drop a dissolving tablet into the tank.
American Standard says not to do this. It may damage flushing mechanism.

The Joy of Green Cleaning. She recommends draining the water out of the toilet before applying the cleaning solution. "If you get the water out of toilet, you can use a milder cleaner with the same squeaky-clean results,"
Simply turn the water valve at the base of the toilet off, flush once, and you're good to go.

11 Easy Ways to Keep a Toilet Bowl Clean Without Scrubbing 18 Things That Actually Make Cleaning Toilets Incredibly Easy How to Always Have a Clean Toilet | Home Cleaning Services in Carmel How to Clean a Toilet Properly - How to Clean a Stained Toilet Bowl | Good Housekeeping

Cleaning porcelain

Is Scotch-Brite safe on ceramic tile? The Scotch-Brite™ Easy Erasing Pad 4004 will not scratch glass, ceramic tile, fibreglass, porcelain, Formica, marble, stainless steel, and most wall coatings.

Can you use a scouring pad on porcelain? After an overnight soak, the lime scale and stains should be easier to remove with the use of a hard bristle toilet brush. You can also use a Brillo pad or heavy abrasive cleaning pad to remove it. The porcelain should not scratch or scar, but use caution just in case.

HoHow To Clean a Toilet - Your Bathroom Cleaning Guide

Make your own bathroom sanitizer, you can fill an opaque spray bottle with one part 3% hydrogen peroxide and one part water.

alternate between using hydrogen peroxide and a mix of baking soda and vinegar which helps prevent hard water stains

Getting Down and Dirty: 5 Toilet Cleaning Tips

The phosphoric acid in soda can remove rust rings and other tricky stains. Simply pour a can of coke around the rim and leave it for an hour or longer.
Flush the toilet and then scrub the bowl with a brush. Flush again.

Products:
Pumice sticks for cleaning hard water stains off a toilet bowl.
LASCO 11-1182 F-10 Shaws Pads Toilet Bowl Ring Remover