Copper paint, CuPro-Cote from LessEMF, is highly conductive and great for painting circuits. According to LessEMF, it can be applied directly onto acrylic, ABS and structural foams, as well as solvent sensitive materials such as polycarbonate and polystyrene.
Awesome, but unreliable for soft circuits, you can try silver paint.
How about trying your hand at making your own conductive paints and glues?
The awesome folks at How To Get What You Want have compiled a list of resources for you!