There are plenty of instructional videos online on "how to pronounce TH" (both voiced and unvoiced). You'll see it really is about putting the tongue between the teeth and not behind them.
I think another problem is that in some cultures, the TH sound is not only unfamiliar, but also comical or even considered a speech impediment. So, I understand that learners think they'll look and sound silly if they use "TH", and try to hide the tongue. However, they end up with D or Z and never sound natural when speaking English. Once they're past that psychological block, their English speech improves.
The major difference to our ears is that D is a sharp plosive, and the voiced TH is a warm continuant - a TH sound simply sounds longer. :)