Floss and Water
If I could go back I'd floss more. And brush more too.
And absolutely ditch soda.
Not kidding kids - at this point in my life nearly half of my mouth is plastic filled, like a bomb of decay waiting to erupt in my mouth as I approach the end of my days.
If you have a few moments of boredom, go check out the Youtube video where they leave a human tooth in a cup of Coke overnight and it disappears by morning.
Most sodas have so much acid in them they start eroding your bone marrow and leeching the calcium out of your teeth on contact for every minute you don't brush after a cracking open a can. My own dentist begged me to take up coffee as a healthier vice to alleviate my caffeinated Coke-fueled cavities.
The array of choices in sparkling beverages has vastly improved since my childhood, too - a can of cherry Bubly tastes nearly the same as a Dr. Pepper to me now. Yet it's not eating away at the last vestiges of my orthodontia (or adding to my middle-aged waistline).
The small bits of food that get stuck inevitably between your teeth - which is where most of my fillings lie - also create plaque and disturbing bacterial growth/decay, hence the importance of floss too.
If I could go back and change just one of my habits as a child/teen, this would be it.
Ditch the soda - pick up the floss.