First, get out any brush you want to use, and select the color you want. But for now, we'll use the color black.
Make a new raster layer and apply your brush.
After you've applied the brushes, use your layer palette to change the raster to something else...(if you dont have your layer palette out press f8)
Now, set the layer to something like overlay
Alright well we've tried overlay....now lets try SOFT LIGHT
You can also try these raster layers with different colors and different raster settings.
Now you can try to make it smooth by going to effects > 3d effects > inner bevel (I suggest when you do this you try any color but black so the smoothness shows)
But overall, rasters can be helpful in MANY ways if you use them right!