sorry for the delay but a busy weekend showing Sat and judging today........
i always start with tiny puppies [ or as soon as you bring them home] when they are laying by you calmly i just lift the sides of their muzzle saying "teeth" as i do it.....once they dont take any notice of me doing it i then move towards the front of the mouth and gently lift upper and lower lips to show the front "bite" if they let you without a fuss then give a treat
I alsodo it when they are in the bath,and any time they are on the table [ obviously only if a table breed!] or any time they are being groomed....try to get as many people, to just handle the pup around the mouth just gently lifting its lips, as you can, always get them to give a treat afterwards.....our postman is a dab hand at "doing teeth"..he even brings his own treats too!....lol....little and often being the key as with everything when training a new pup....never get cross, if it it doesnt happen, try a few mins later.
hope this helps?
Pauline