The basic idea behind long and short stitching is simply to vary the starting and ending points of the stitches so that you can get a smoother color shift and to be able to easily stitch contours. I've made three sample shapes with long and short to show you a couple ways to use it. The first thing to do in crewel embroidery and (thread painting too) is to outline your shapes with split stitch or split back stitch.
First shape is a basic box. The first row of stitches is composed of some longer and some shorter stitches. For the rows following, you use only long stitches (except at the end of the shape, of course), but this isn't a precise science, so the stitches will have some natural variation in length anyways.
The next shape is a flower petal. Another thing to keep in mind, is that it's important to work from the outside edge inward. This example shows how you may need to change the angle of your stitches to get a nice shape.
The most challenging (and fun!) thing to do is fill complicated shapes that bend around curves. This is particularly useful when filling animal shapes.
Here you see all three shapes filled in. I have limited colors on hand, but I think that you can still see the potential this stitch offers.
I hope this lesson was helpful in demystifying long and short stitching just a little bit. Give it a try! It's not so hard as it seems. :)