This tip is from Jennifer Blenkinsopp at ArtWanted.com:
With pastels you have to build layers. For instance, with the Isle of Arran, the grass in the foreground only needed two layers of two different greens. I left some of the paper showing through and did not blend, in order to give a rough look.
With trees, I usually start with darks, followed with light on top. Buildings are done with light strokes, with some paper showing through - and blended...
All the commonly available art fixatives are much the same. If you can get away with it, don't fix your work... If, however, you are building layer upon layer, then you will need to fix between the layers - but not the last one. I have used hair lacquer in the past too but I found that it darkened the painting more than purpose made fixatives do.
So all it is, basically, is to build layers. When you think your work is getting a bit muddy, 'fix' it.
Once the fixative has dried, you can add another layer of pastel. I do not fix my final layer.