To be successful at brushing your dog's teeth, you must make it a positive experience for both of you. Make the experience positive by praising your dog throughout the whole procedure, with reassurance through every step.
Remember, even with a diligent at-home dental care routine, adult dogs should have their teeth professionally cleaned at least once per year. If your dog has loose teeth or seems sensitive around its mouth, or is bleeding orally, consult your veterinarian.