I have problems differentiating between d and j as initials. Note: I am from Denmark, not the USA. I had hoped for some ‘tricks’ regarding this in "Say it right:, but alas not so.
D and J would have a different tongue position. For D, the tip of your tongue would be at the roof of your mouth. For J, the tip of you tongue would be at the base of your lower teeth, and the rest of your tongue would kind of arch upwards towards the roof. It’s unintuitive for English speakers–maybe it is for Danish speakers as well.
I found the following blog series to be a good resource for pronouncing the various initials: http://www.sinosplice.com/learn-chinese/pronunciation-of-mandarin-chinese