'Make a bed' with your fists- Palms toward you, push fists together and put thumbs up as 'bedposts'- The left fist now looks like a 'b' and your right fist looks like a 'd'
Also helps in 'fancy' restaurants- your 'b' thumb points to your bread plate, your'd' thumb points to your drink!! Prevents using the wrong dinnerware!!
I had a few Kindergarten students with this same problem. We drew our 'b' and 'd' as pictures. The 'b' was a bear's belly and the 'd' was a dinosaurs back. We had fun drawing the pictures and it was mentally easier for the kids to remember with the picture/word association.
yes, def. developmental. I have had second graders continue to battle with this problem. I do the " draw a bat I and then add the ball o that will make a b. I tell them to play baseball first you need a bat, then you need a ball. for d just tell them first you nee a drum o, and then you need a stick I making a d. I write it down and let them keep it with them at all times. Good luck. It is good that you are aware, but don't worry too much.