Simply draw or imagine the word "bed" in lower case letters as an actual bed. A bed requires two end posts and a place to put a mattress. The post of the "b" at the beginning of the word and the post of the "d" at the end of the word act as those bed posts and the body of the word creates a surface on which to place a mattress. Since the word "bed" contains both letters in question, it is easy to compare them and immediately see which is which. See the simple image included. Young children may benefit from imagining somebody sleeping on the mattress.
When one of my children is confused with "b" and "d", I say, "make your bed" to remind the child of this imagery and he or she figures it out right away. This type of imagery empowers children to learn and remember on their own rather than simply giving them an answer they can easily forget.