In the 21st century, you could go into a dentist’s office and have your teeth reconstructed. Teeth are made of a protein that has nucleation centers every 200 nanometers to trigger the growth of hydroxypatite crystals, and the rocklike hydroxypatite itself. The tooth protein was sequenced in the late 20th century, and the chemistry of the hydroxypatite was understood long before that. So in the 21st century, your dentist simply cleans the tooth, programs a small CAD-controlled machine with the desired tooth shape, tells you to say "AAH" and in a few minutes your tooth is better than new.
Unfortunately, dentistry hasn’t made it to the 21st century… most people still receive dental treatment developed in the 1840s. You go in to the dentist, he makes a bigger hole out of each cavity, then fills it with mercury amalgam. The amalgam expands and contracts more than the tooth around it, it conducts heat and cold far too well, and as a bonus it releases traces of mercury. The dental journals actually recommend not getting fillings at the first sign of small cavities, because they weaken the teeth.
So, the logical thing to do is… not get cavities. For most of us that would require time travel, so that we could get our public school vending machines to give us green tea instead of phosphoric acid carbonated drinks, vegetables and fruits instead of Twinkies, etc. Time travel may actually work… but it would be very expensive.
But if you still have some teeth, there is a way to build up more hydroxypatite on damaged or worn tooth surfaces. A cream containing sodium calcium phosphosilicate will release the correct ions when it hits saliva to rapidly deposit hydroxypatite. This coating of toothlike material reduces sensitivity of worn teeth as well as stopping the growth of many tooth bacteria; it’s like cement overshoes for Streptococcus mutans. (Bacteria on teeth and gums affect heart health as well as the attractiveness of your smile, so this is not a small benefit).
You can read a paper on a clinical trial of sodium calcium phosphosilicate here.
You can get this stuff from Novamin.com. It’s good insurance; after all, sharp teeth are the only weapons Americans can carry in airports, schools, East Coast cities, and other likely terrorist targets.