Photoshop is an extremely powerful graphic application as evidenced by it's extreme popularity among both artists and developers alike. One of Photoshop's more (ab)used features is the drop shadow Blending Option. For certain things the drop shadow works just fine, but if you're looking to create more lifelike shadows it just won't do. Before jumping into the tutorial, there are a few general points to consider:
Keep Your Light Source in Mind
I'm starting with this point because there's nothing that will kill the realism of a design faster than having shadows going in different directions as if they are being lit from random light sources. It's usually best to stick with a single light source and have your elements cast shadows based on it. For example, if your light source is top-left, your shadows will fall to the bottom-right. Having one element cast a shadow the opposite direction can make the entire layout look wrong.
Keep the Object's Height in Mind
If your desk is as messy as mine (or even if it's not) you'll probably have several objects of different heights. Notice how different their shadows are -- you'll want to replicate this in your layout in order to make your objects seem like they belong. The further away from the surface an object is the fuzzier it's shadow, and vice versa. This is very simple to emulate using Photoshop.
Ok, Let's Get Started...
For this example I'm going to use a simple objects: an "information" sign that I drew using vector shapes in Photoshop (feel free to download the PSD file by clicking here.) If you click the thumbnail to view the full-size image you'll notice that the sign looks, well, fake. There's no depth to it at all. Fortunately, a simple shadow can solve the problem and help define the height of the sign relative to any other content on the page.
At this point there are two ways that you can go: 1) either clone the sign itself, flip it around, and stretch it to it to get it to approximate the actual angle of the shadow, then turn it black using a Blending Mode or; 2) draw the shadow by hand using the Pen Tool. There are good times to use either method but in this case it's probably best to just draw the shadow yourself because the sign is at a bit of an odd angle so it would be difficult to stretch it the right way.
Here (download) I've created a black vector shape to match the shape of the sign. Note that it doesn't have to be exact because we're going to blur the shape and apply a mask -- the key here is to get the angle correct and that gets back to understanding where your light source is in relation to the sign. In my case, the light source is coming from the top-left and above and behind the sign causing the shadow to fall down in front of it.
Here's Where the Magic Happens...
Obviously this doesn't look any more realistic than when we first started, and that's because the shadow just looks like a black blob. So the first step is to blur the shape. Click Filter/Blur/Gaussian Blur and set the Radius to 3 pixels. Click OK and you should have a reasonable looking shadow.
But it's still not done because we haven't taken into account the fact that the further away the shadow is the less clear it will be. To solve that problem we need to apply a layer mask and gradient. Click the shadow layer to select it and then click the Add Layer Mask button at the bottom of the Layers palette. Now click the Gradient Tool and select a black to white gradient. Start the gradient from slightly beyond the end of the shadow and drag it toward and through the sign. You'll need to experiment to get the best results but eventually you'll wind up with something like the image at the right.
Now change the opacity to something in the neighborhood of 35% and your shadow should be looking pretty realistic.
One of my favorite tricks is to put a little circle around the base of the sign which makes it look like it's modified the background -- a hole in the ground, in this case (download). As I mentioned in my last article I like to keep things simple so I wouldn't want to overdue it with this image. But as an example of how to expand on this, you could always create a grass or pavement texture and have the sign stuck in as if it were on the side of the road. However you use this technique, just make sure to keep the two key points from the beginning in mind in order to create realistic shadows.