I worked for several years teaching phonics to struggling readers in a private tutoring practice. About a third of my young clients came to me with a b/d reversal problem in spite of various things being tried by their teachers in school.
In my readings, I came across a method described by Romalda Spalding back in the 1950's that made a lot of sense to me, so I started telling it to each of my clients who still confused b and d, either when reading them, or printing them, and found that it worked within a couple of sessions to correct the problem.
I think it's because the method involves placement of lips and tongue, something they're doing anyway as they read, and it could be learned easily. There's an old post in here where I mention it, but it's buried within a comment to a question, so I thought I'd just post it as an article instead.
It works both ways, whether reading or writing b or d, and involves making a line with your lips when you see the line first (the "b"), and a circle in your mouth when you see the circle first (the "d").
I would have the child look in a mirror, say the /b/ sound, and point out the more or less straight line they were making with their lips. "When you see the line first, make the line and a /b/ will come out."
Then, for the circle, I'd have them say the /d/ sound and imagine a gumball in their mouth, pointing out how their tongue was curled against the front of their mouth behind their teeth. "When you see the circle first, make room for the gumball in your mouth and a /d/ will come out."
The above is about all I really did, and it worked. This is getting long, but with proper preparation, it works for writing b and d also. You can see more at Telling b from d