Based on advice from a professional bra fitter, I went down a band size and up a cup size.  This formerly 36C petite person bought this bra in a 34D (based on good reviews).

WOW.  I have never felt so supported by a bra in my life, and am also finding that although I could probably go DD, this bra holds me in and and up.  I used to only wear padded bras, to keep my shape from sagging, but don't need it with this one.  I feel light and not weighed down now that I've got a correct size and good support - it's great for my back.  I'm ordering two more right now.