Think of the word "bed".
Imagine it as a bed, with the mattress resting on the rounded parts of the letters b & d.
The letters have to go in the right order, or the mattress will fall down.
Hope this helps.
I use solarjunkie's bed, but you can also try the bat and ball for a b. The bat is the downstroke/upstroke (dependingonhow you form the letter) and the ball forms the round bit of the b. Difficult to explain! Don't have a similar one for the d.
hello, i often stick a small b and d to the child's desk with pictures around each of items that begin with that letter. sticky back plastic will hold it on unless you have an over zealous cleaner!
I teach 5-6 year olds and I always say that b's look as if they have big bellies and tell them to remember b for belly. It seems to work most of the time!