The most common cause is just simple bacterial overgrowth. A real thick coating can indicate thrush, a Candida overgrowth.

Have you tried a tongue scraper?

You might also want to try drinking some kefir.

here is an experiment: eat whole steak and then check your tongue-no more white tongue.
if you eat alot of starchy food, not only the tongue appears white, it also feel thick, that means your pancreas is at risk. to proof that is true, check the zyphoid process and at the same level both sides rib, all have pain points.