The dentist (first time in 15 years!)

[Saintly]

mouth germs eat sugars and make acid which eat you teeth
food acids cause erosion - main from citric acids, sports drinks etc

you remove the food.
there are red disclosing tablets which dye the plaque red, therefore showing you where your teeth are skanky and therefore need to be cleaned better.
circular motions or flicking from the gums with a small head and soft bristled toothbrush and FLUORIDE toothpaste - as when you clean your teeth you are disturbing the plaque and then putting fluoride over the tooth surface to strengthen it

OK, that's too descriptive :P just as well I'm not the one with the problem (remember I hadn't seen a dentist for the first 30 years) and the first time I saw one, it was a bi-annual clean (my wife complained that I'm not utilising our health cover)