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While Photoshop cannot bring a completely blurry image back into focus, it can help most of us improve the sharpness of an image with the Unsharp Mask filter. For example, there's no hope of undoing the inherent blurriness in this picture, but you can make out a little bit more detail on those two little wiggly kids using Unsharp Mask.
Pull down Filters to Sharpen, and then choose Unsharp Mask. It previews your adjustments, but usually setting the amount to between 80 and 120 will be enough sharpening to bring out the crispness in your photos. The Radius and Threshold settings at 1.0 pixels and 0, respectively, is fine for all files.