This video tutorial, by photoshopuniverse, shows you how to reduce the depth of field after a photo is taken using Photoshop. This enables you to ensure that your main subject is clearly in focus while the background is out of focus or blurry. First duplicate the layer using the short cut that was mentioned and apply the filter lens blur to the new layer and adjust the radius so that an adequate amount of blurring is achieved. Add a layer mask and bring the subject to focus by selecting the brush making sure it's on black and white expose the subject while varying the brush size as shown. Fill in the holes you missed by Alt + clicking on the mask and filling it up with the brush.