Here's a little trick that helps my kids distinguish b from d. Once kids are confident working across a line of text from left to right, they are ready to "bulldoze" from left to right. (If they need help remembering to start at the left a green sticker, for go, at the left and a red sticker, for stop, at the right often help.)
As they approach a b, they will find that you can bulldoze a b.
Many kids who have started to get frustrated by issues distinguishing b and d really enjoy this activity.
If kids try to bulldoze a d, it just tips over . . . .
Once this activity is introduced in a small group, kids enjoy independently "bulldozing each b" during literacy center time.
Each time they bulldoze the letter b into a new box they say, "You can bulldoze a b!" It really sticks with them!
MatchBox and HotWheels often have small, inexpensive bulldozers available. This one belonged to my son and I "inherited" it when he grew a bit old for it.
Many of my kids love this activity. I hope you find it useful!
P.S. - Do you have favorite ways to help kids distinguish b and d? I'd love to hear about them!