Embroidery Source has a factory outlet in Fairfield (205 Fulham RdFairfield, Victoria, 3078 Australia) where they sell "Commercial Quality Embroidery Products At Warehouse Prices ". I discovered this place when I was studying textiles and what a find!! They have machine embroidery threads in every colour (more than you'll find in Spotlight or Lincraft or most other sewing shops) and on bigger rolls. As they say, they are commercial quality and therefore most of what they sell are the bigger spools, but these are still workable on your regular sewing machine.
But I digress. I started out using Solvy which is a plasticy film that dissolves in warm water. I used this to do the sunflowers and wheat stalks in these two pieces.
Solvy is fairly flimsy however and when used as I did (stretched in an embroidery hoop) it had a tendancy to stretch and tear.
Water soluble vilene on the other hand it like working with fabric. It's much stronger and doesn't stretch or tear (unless you're trying to tear it). It handles heavy machine stitching and because it is fabric-like and white, you can draw your design directly onto it and the lines disappear when you dissolve it (as long as your lines are too dark).
I stitched this leaf using water soluble vilene. I use 2 layers because I don't have a backing fabric and I think that it's more stable for heavy stitching.
I didn't wash the vilene out completely because the residual acts as a fabric stiffener. I just wash it enough so that there aren't any white 'gluey' patches left.