Lacey at 2 years old, had never had her teeth cleaned by a vet or by the breeder. Her breath was so foul when I got her in December that I couldn't stand to have her anywhere within a 10 foot radius of me. My vet said her teeth were TERRIBLE "as nearly all Chihuahua-teeth are". Her back teeth were BLACK. That's right; BLACK.
Determined to save this girly's breath and general tooth health, I went on a heck of dedicated cleaning spree.
I started brushing her teeth daily. There is a liquid tooth-cleaner to put in their water, and I used that as well. I also used the foam-cleaner (one squirt at the back of the mouth on each side) daily with her. Just 2.5 weeks of this nutty regimen, and her teeth were looking SO much better, and her breath didn't smell like she'd had her face glued to her bum all day.
While she fought the toothbrush for the first couple days, all I have to do now is plunk her on her back and say, "Torture time my little lady! OPEN WIIIIIDE!" (hahahaha) and she'll lie there contentedly while I brush her (getting there) pearly whites and squirt the foam in her mouth.
I still use the foam daily and have cut the brushing to 2-3 times a week. She's also learned to like Dentastix mini chews, which appear to be taking a lot of the surface tartar off and helping control new build-up.
I apologize for the semi-long post, but I wanted to share what's been working for us!