We have come a long way in the tooth brushing area in the last couple of months, thanks in large part to the book ABC Dentist and the advice of a few friends (namely, get a book and insist on tooth brushing). We were intermittent brushers, and now we are religious with both kids, twice (or even three times) daily. If I forget, they remind me.
In this book, the authors run through the alphabet and use each letter for a dental office item. The items they choose are relevant and informative, and interesting to my preschooler. The drawings are realistic and the whole book is just enough to keep us interested but not too scary (we actually reword "Q" for "Questions"-- it gets a bit scary-- and I gloss over "O" for "orthodontist"). This book has helped my older child understand the need for tooth brushing, and he is a great influence in this way for his younger brother.
We use Weleda Children's Tooth Gel for them and they like the mint flavor and I like that it is non-toxic!