Have you ever wanted to give even more depth to your LOs by leaning an element against another element instead of stacking them on top of each other like most clusters are? Have you ever thought "How weird is that drop shadow outlining the tree instead of making the shadow on the ground? Never seen that in real life." Or how about you've got a beach scene and you want to put some shade under that palm tree or beach umbrella?
See the shade under the umbrella and over the beach chair, but the more shadow-y shadow behind the Beach Bum popsicle stick and sail boat?
This one shows a more realistic shadow giving a little depth to the cluster like what you would see if the light was coming from the extreme bottom left corner as though it was late in the afternoon instead of from above as though it was 1:00 in the afternoon.
By no means am I an expert, and I'm not much of a teacher, so definitely ask questions if I missed a step or confused you somehow. I'll give instructions for PSE9 and assume you have just started using PSE, but it should work with any PSE and I think it should work with Photoshop as well (I'll have to check it later, but PSE is what I'm most familiar with). This is something you have to play around with to get the look you want; it's not an exact science so I can't give you the exact settings to use. That's up to you to decide what looks most realistic.
Working on the Popsicle Stick style shadow:
Step 1. Duplicate the image you want to shadow. (Layer -> Duplicate Layer, or you can left click and choose Duplicate Layer from the pop up box. If you are like me and don't feel like moving the mouse that much, the keyboard command is Ctrl+J.) The duplicate will be above the element you are working on. Name it shadow if you have to in order to keep things straight.
Step 2. Make sure you are still working on the shadow layer and click on "Enhance -> Adjust Color -> Adjust Hue/Saturation". The pop-up box has 3 sliders. You want to slide the bottom Lightness one all the way to the left to -100 so that the shadow is now completely black.
Step 3. It's too harsh to be a shadow right now. To solve that, click on Filter -> Blur -> Gaussian Blur and adjust it until the edges are softened enough. For shade you want more Gaussian blur, for a shadow, you can use less.
Step 4. You want to stretch the shadow now for that late in the day look or to make the shade spread out from under your umbrella. You will click Image -> Transform -> and then.... I play. There's 3 options that will give you the effect you are looking for. Skew, Distort and Perspective. Choose one, grab the corner of the box that appears and move it until you get the look you want. It will still be too dark, don't worry about that. Just play until the shadow looks like it's at the right angle for the effect you are looking for. If you get to a point where it looks really messed up use Ctrl+Z to undo!
Step 5. Now, something's wrong... your shadow is on top of your element. Easy peasy. Move the shadow layer below your element layer. Looks better?
Step 6. Your shadow is still too dark to be a realistic shadow, right? More playing! You can adjust the colour slightly to a dark grey by clicking on the little foreground/background colour boxes and choosing a dark grey from the pop-up box. I prefer a very dark grey instead of black.
Step 7. To make it a more shady shadow, adjust the opacity. I dropped it right down to 18% for the popsicle stick.
For the shade under the umbrella, I pretty much did the same thing except the Gaussian Blur is larger. I still played with the Skew/Distort/Perspective, and I still played with the opacity.
For the perspective shadow under the African masks and spear, I followed the same procedure. I just played with the skew thingy a bit until I got what I wanted.
I hope this helps somebody!