Yup, you've got some nice depth going on in your picture already. But don't think you're done yet. Many beginning CG artists use only one layer of shading, which forms only marginal shadows and gives the image not enough depth. To add more realism to a picture, you'll need to add on to the shadows. Darker, more layered shading equals more contrast equals better realism, but don't reach for the black paint just yet. That's why we decided earlier to not use the Burn tool, because it just darkens a color mindlessly, and in the end your objects will just look, well, burnt.
Now, you can deepen the shading with all the same color, and you can get away with it just fine. But to make the picture more visually appealing, try adding tints of different colors, and let them show through at different places. In a shadow cast by dark clothing, the color may be more bluish, while the dark side of an arm may be a reddish tint. Click here to see a comparison: the picture on the left was shaded only by overlapping Multiply layers of the same color, while the right picture had three shadow layers of peach, gray-blue, and light red. The darkness of the shadows are about the same, but which one looks more interesting?
Okay, so using colors is nice. But what colors should you pick? Not to worry, you have plenty of room for creativity here. Since you're applying the color on a Multiply layer, it will be combined with the color below it, so you won't have to worry much about how well they'll blend. If you feel your shading color is standing out too much, lower its saturation, or in other words make it "more gray". You can also apply it lightly by either raising the lumination ("more white") or lowering your Brush's transparency. While it's a good idea to have some general consistency in the lighting effects of your picture (e.g. don't have a strongly reddish shadow on one object while the darkness is green-tinted elsewhere), it's hard to really "go wrong", and of course there's always the lovely Undo button. Be brave!
Some general guidelines: A colored light will cast a shadow of its complementary color. For instance, a red light will make a greenish shadow, and a yellowish light, which is most common due to natural sunlight, will leave blue-purple shadows. Also, keep in mind that objects next to each other will reflect light onto each other, thus influencing their colors. A white ball sitting on a green cloth will have some greenish tint on its underside, for example. The exact amount of color an object picks up from its surroundings would depend on its own color and material.
Yup, you've got some nice depth going on in your picture already. But don't think you're done yet. Many beginning CG artists use only one layer of shading, which forms only marginal shadows and gives the image not enough depth. To add more realism to a picture, you'll need to add on to the shadows. Darker, more layered shading equals more contrast equals better realism, but don't reach for the black paint just yet. That's why we decided earlier to not use the Burn tool, because it just darkens a color mindlessly, and in the end your objects will just look, well, burnt.