I add 2 tablespoons of Bragg's Apple Cider Vinegar to a glass of warm water each morning, coupled with some honey (for taste).
I specifically use Bragg's because it contains what's known as the 'mother'. This is a sediment that has natural chains of protein enzymes.
If you do use it, I would recommend you drink it through a straw, it's quite acidic and is not going to be good for your tooth enamel.