Two things I do which may help:
1. I use neat PVA to lay a thin (3-5mm)boundary strip of neat ballast along side the sleepers overlapping the cess (which I generally lay prior to ballasting). This helps set the boundary when dry-ballasting and helps hold the shoulder in place. I have used masking tape for the outside of the glue strip when I want a particularly neat finish. I think I picked this up from a US magazine.
2. I use the wet water very fine spray once the rest of the dry ballast is in place and I now use matt medium rather than dilute PVA for the final step(but both work).
This is my latest effort...you'll notice there's no cess.....doh..
I also do a bit of an overspray once its down. (Phoenix Precision weathered sleepers..)
Chris