Bb and Dd reversals are common in the younger grades and I have always found that one of the ways in which to assist students in not only writing these letters but identifying them in words as well is to create a visual that sticks in their minds. For many students the little sayings "b is a bat then a ball" and "d is a donut then a dollar" help them to distinguish which comes first, the straight line or the curved line. The pictures that are added behind the letter formations really help those visual learners see what it is they are saying. I post these next to the main whiteboard, which is also typically near my alphabet strip to really help students distinguish between these two letters. The page can also be shrunk when copied to create smaller versions which can be attached to desks or pencil boxes for students who are frequently consulting the posters. The goal is that by using the posters they are increasing their frequency of correctly identifying and writing the letters and will rely less and less on the posters themselves.