I brush Teddi's teeth every night! I started as soon as I found out that yorkies were prone to tooth problems. I bought a peanut butter-flavored toothpaste (sorry-- forgot the brand name) and a toothbrush "for puppies" (it has a wide handle area so it's easier to keep the mouth open; it's not a finger brush).
The first time I brushed his teeth, I first let him lick the toothpaste so he would get comfortable being close to the toothbrush. Then (I think-- I don't quite remember!

) I pretty much just held his mouth open and gently brushed his teeth. His tongue was all over the place! I had to do some maneuvering to get to his teeth!

At first he would run away when he saw me with the brush but not anymore... I guess he got used to it!
You gotta be careful though... Their teeth can be sharp!