Shadows are an essential skill for any one making composites in Photoshop. This one is quite good, but wrong, because it does not parallel the other shadows in the scene. It is OK as far as her waist, but it swings too far let after that, because her body is vertical from the waist upwards, so the shadow should align with other vertical objects in the scene.
A good book on the subject - actually it is an excellent book - is Matt Kloskowski Compositing Secrets.
Several ways to approach shadows. You can use Layer styles > Drop shadow, and then right click the effect and chose Create layer. That produces blurred black silhouette. Then use Free Transform, and Ctrl/Cmd drag the center top handle to place it.
The way I do it is to Ctrl/Cmd make a new layer, which opens a layer below the subject. Then Ctrl/Cmd click the subject layer to load its selection, and Alt/Opt backspace to fill the shadow layer with black (which should have been the foreground colour). Then use Gaussian blur, and layer opacity after using the same Free Transform steps as above.
Of course neither of these are going to work if the light is coming in from the side, because the shadow outline will be wrong, and that’s why it can’t be done with plugins and automatic processes. It HAS to be done manually. What I do in this case is either rough out an outline, or use Google images to find an appropriate shape I can cut out. Resolution is not important so you can up-res with impunity. Shadow length can be worked out from other shadows — if they are there.
It gets a tiny bit tricky with steps walls and buildings, but it is not hard to do. Put a vertical shadow from the base of the kerb (or vertical surface) which will show you where to continue the rest of the shadow.
I could go on for ages, but you get the idea. I generally use two or three layers for a single shadow because they are hard close to where the subject meets the ground, so I do that separately. Sometimes shadows get more transparent with distance, which is easily done running a gradient along a layer mask. This generally takes a few goes to get right, so I use Foreground to Transparent (the send option) which lets you build the effect.
You can probably find a PDF of Compositing Secrets, but please buy it if it is useful. You should also try Google Images with this sort of search Note how long shadows have perspective, and that the fade is mostly very subtle. The eye picks up on this sort of thing, and the brain knows when it is wrong intuitively.
Blimey, I’m on a roll, so one last thing I’ll say…
Shadows are also important within the subject’s boundaries, and so are highlights. What I do is make a work path that follows the intersection, and stroke the path with an appropriate sized soft brush on a new layer (of course). Then Ctrl click the path in paths panel to load its selection, and add a layer mask to the shadow layer. I nearly always use Gaussian blur but you have the Shift the Layer mask to turn it off before using blur, or it blurs the edge as well.
OK, just a little picture to illustrate a way to use this.
This picture of a Strikemaster in a RNZAF hanger was being lost against the background because there was so little contrast between the outline of the aircraft and the BG. So I traced the outline with the pen tool, ans troked the workpath on a new layer with a thin white brush. I then blurred the white line, Ctrl clickr the workpath to load it as a selection, and added a layer mask. The resulting highlight can be passed of as a specular highlight because of the nflat angle between the outline and the high windows behind it.
