ZiwiPeak sells some antler chews that are really great for cleaning teeth. I like the ZP brand because they cut them longwise, exposing the marrow, which provides really nice scrubbing action. There are other brands of antlers out there though if you look around.
Aside from that.. I use Leba III spay. I can honestly swear by this product even though it's pricey (about $50 per bottle, but I've only bought it once and it's lasted me about a year). It's an enzyme spray that dissolves plaque. You just spray it in the dog's mouth (anywhere it can mix with saliva is fine, it doesn't need to be directly on the teeth) without any food or water 30 minutes before or after. Twice a day for getting rid of plaque, and once a day for maintenance. The dark plaque on Teddy's teeth softened after a few days of using Leba III into this REALLY foul-smelling, soft goo that could be easily scraped off with a fingernail or a toothbrush. I got into the habit of spraying him once in the morning, waiting 30 minutes, brushing his teeth with a toothbrush and warm water, and then spraying him again in the evening. After I brushed the yellow goo off, his breath smelled so much better and his teeth were nice and white.
I wish I had a before picture
but here's how they looked after using the Leba III for a while:
But I highly recommend brushing your chi's teeth alongside any chews you can provide, even if it's just with a soft-bristle children's toothbrush and warm water. Adding in the Leba III would give you much better results though, obviously. And like I said, you can't go wrong with antler chews.
They last for a very long time too.