Uliss has had her teeth brushed since she was a tiny puppy, she's now nine years old. I buy an ordinary baby toothbrush from Boots (I find it more managable than the pet ones) and use chicken flavoured toothpaste from my vets. Both my dogs love having their teeth cleaned. One of the tricks is to squash the toothpaste down into the bristles otherwise your puppy will just eat the blob of toothpaste!
Best of luck, I know some people have problems cleaning teeth but mine took to it straight away.