Here's a way to help your students know the difference between lowercase b and d. Follow the directions for writing the letters in the illustration below. Before writing on paper, have the students say the directions as they write the letters in the air. Once they've done that a few times, I would have them follow/say the directions and either they write each letter on the chalkboard with a wet finger or write with an erasable marker on a dry erase board. They should write only one letter at a time at first. When they've got the directions down, they can write the the letter b and then the d side by side. Tell the students that they will know if the letters are written correctly if they look like they will bump bellies.
However, if they write the d first and then the b, the letters will be back to back.
The important thing is, if the students can remember these little sayings (and follow them)while writing these letters, they will have conquered the b and d conundrum.