I guess starting as young as possible. So when they're a puppy, getting them used to the action of brushing their teeth. You only have to brush the outside, so you should get them used to even just a finger brush on the outside. If you do it right before dinner, they'll be really excited about it because they know that dinner's coming afterward.

Ideally, every day, but every other day will suffice if you can't. At least every other day.