I have a new boxer puppy and a 5 year old lab. I started brushing my Labs teeth when she was a puppy. I started with just my finger and some dog toothpaste (don't use Human toothpaste, not good to swallow). I think they even sell finger brushes. I worked up to using a regular toothbursh. My lab will even let me scrap the tarter off of her teeth. The last time I was at the vet, he said that he had never seen such clean given her age. I plan to start getting my new boxer puppy started asap.
I just have my lab lay on her side and I hold her gums back and brush. When I am done she always gets a treat for being a good girl and lying still. My lab trusts me 100% and that is important. It will help if you dog has been exercised and ready for bed.
I hope this helps a little