Dracula drank Drano.
That was my tongue twister for "D"! This one isn't my best, but my Mom can't say it very well. Here's another tongue twister that I found on the TGIFriday kid's menu a long time ago. This was one of my inspirations for making up tongue twisters when I have nothing else to do:
any noisy noise annoys an oyster
When me and my sister were first learning it, we kept on saying, "any noisy noisy noisy noisy noisy noyster." It takes some time to learn, but I think it's a pretty good one.