Natural soap plus a few drops peppermint oil

Buy a bar of unscented very natural soap that lists its ingredients on the label as being lye, natural oils, and not much else, WITHOUT glycerine. glyerine coats the teeth (and is one reason why commercial toothpastes are bad, they usually have glycerine). Your health food store will have the natural bar soap I mean.

Rub barely a tiny swish of the soap on your wet toothbrush (barely any), then drip a few drops peppermint oil on the same toothebrush. Voila.