An old-fashioned idea from tailoring: use a woven interfacing, something like buckram but not so stiff. Cut to size and lay on the reverse of the brocade. Using large basting or whip stitch, hand sew through the woven interfacing and just catch some of the threads of the reverse brocade. Do this in rows, following the contours of the pattern. Try not to leave any large areas unattached. You can then proceed as usual.
This handwork is traditional in fitted menswear and women's fine suits and coats. I hope this will be helpful.