I have a book called Toe by Toe that I used for my (slightly) dyslexic dd1 when she was in year 3/4 and that illustrated the b d confusion with two pictures: a bat (on the left, forming the upright stroke of the letter) and a ball, making up the letter b; and then a ball with the bat (on the right) making up a letter d. The line underneath read "Bat before ball says b".
We found it quite effective as it's short and snappy and they can easily visualise the ball and bat.