Good question Kyle! it's so difficult with Pixie as she's very tiny and has a very tiny mouth! it's a bit of a struggle so i have to be patient..I use a cotton cloth wrapped around my index finger and smeared with a little bit of doggy toothpaste...I don't think i would ever get a toothbrush in there..or a finger brush! i'm hoping she will eventually get used to it and let me get further back there to reach those back teeth..i'm also going to be purchasing some petzlife spray & gel and give that a go. I have also been giving her fresh raw bones from the butchers..they have helped to dislodge some of her retained baby teeth so i'm hoping it will also keep some of that plaque at bay too!