I have been poking about the net digging up a little information on surface design techniques for textiles. Here are few links I have dug up.
Ruth Issett has provided instructions on using oil and wax crayons on fabric in Make your Mark use Markal Paintstiks to create rich surfaces for embroidery. The article illustrates masking, rubbing and mark making techniques on fabric, all of which make an interesting jumping off point for projects.
Stamping Fabric covers applying stamps to textiles using easily obtained supplies and fabric paints and inks.
Pamela Watts describes a technique for combining fresh flowers with stitching by sandwiching them between bondaweb.
Adding a metallic foil to fabric can turn a surface that is quite mundane into something special. This technique uses paper-backed fusible webbing to adhere the foil to fabric.
Finally this article by Janice Hay on creating interesting patterns on fabric by embellishing a mono-printed foundation fabric should provoke some ideas too.
All of them are on my list of things to try on fabric postcards.