My 4lb Bitsy can be a squirmy handfull at times, but after the vet showed me how to brush her teeth, I've had no trouble and do it every night before bed. I put her on top of the bathroom vanity and as she watches I put a little poultry-flavored toothpaste on a tiny toothbrush and then I say, "Let's brush your teeth! Smile for Daddy.", and with the thumb and index finger of my left hand around her muzzle, I gently pull up her gums from the sides and quickly brush her teeth. She will give me about 10 seconds before she has had enough, but (IMO) that's enough time to brush. I'm hoping that the enzime toothpaste also continues to work for a while afterwards. Since we have been doing this nightly brushing (2 months now), she knows it is coming and when I say, "Smile for Daddy!", she has recently started showing me her teeth. Eventually, I may not have to open her gums with my fingers at all.
Now, if I could just get her to be still for a nail trimming...