Step by Step guide
1. Scan at 600 dpi, use PS to downsample to 300 dpi
2. Run despeckle filter once on each colour channel (red, green blue)
3. Use the quick mask tool to paint in areas of skin your want to smooth. In quick mask, use black colour to paint in your selection, white colour to deselect. Use a soft brush to feather at th edges!
4. Save your selection as an alpha channel. I call this selection "skin".
5. Next, use a hard-edged brush to paint away areas you don't want to smooth, like eyes, eye lashes, outline of the nose, lips, and other skin creases etc.
6. Save this selection as an alpha channel. Name this selection "masked".
7. With the masked selection active, float in to a new layer (CTRL-J).
8. Use the gaussian blur on this new layer (radius 14 pixels). Quite a high setting, but play with the settings till you get the effect you want. Usually, the bigger the scan, or the bigger the skin area, requires a higher guassian blur setting.
9. Set opacity of the blurred layer to 70%. Check to see if effect is pleasing. (Do not overblur and kill detail!) The best effect is still to see a little of the skin detail, like pores, etc.
10. Save your work and downsample to 95dpi.

After the steps above, this is what you get:

Step by Step - pt 2
Now that your pic is at a smaller size, it's time to run unsharp mask to bring out the details!

1. I ran unsharp mask at settings (25%, .radius 0.5 and threshold 0%) 4 times. Remember not to oversharpen!
2. Next, I loaded in the "skin" selection, and ran further unsharp mask at this settings 1. 25%, radius 0.5, threshold 0% 2. 25%, radius 0.3, threshold 0%) 3. 15%, radius 0.3, threshold 0% 4. 8%, radius 0.3, threshold 0%
3. It's better to run the unsharp mask filter multiple times at smaller settings rather than to run it once at a large setting.
4. Select the background behind the head, and floating it on a new layer and ran a guassian blur filter on it to smoothen the grain arising from the sharpening process.
4. During the sharpening process on the base layer, you'll find that the 2nd blurred layer is nicely masking the grain of the skin arising from the sharpening  
5. As an illustration, here's what the sharpened pic looks like without the smoothing layer:

Step by Step - pt 3
Step back and take a look.

1. decided to bring out a little more skin detail by reducing the opacity of the blurred skin layer from 70% to 65%.
2. Added in text, logo and border.
3. Finished!

步骤一：
1.扫描@600像素，用Photoshop缩小到300像素(图像大小)
2.在各个通道里(红，绿，蓝)，使用滤镜了里的去斑(降噪)各一次
3.打开快速蒙板，在快速蒙板里用黑色画笔选择出你想模糊的区域，用白色去取消出选择的区域。用软边画笔(偶不太清楚正确的叫法是什么)扫过轮廓。(偶自己的理解是，在快速蒙板中，PS默认的颜色是红色的，用画笔选择出你不想模糊的区域，橡皮擦擦掉想模糊的区域)
4.将此选区保存为alpha通道，更名为skin
5.用硬边画笔涂掉你不想模糊的地方，例如：眼睛，眼睫毛，鼻子的轮廓，嘴唇和其它的皮肤褶皱，等等。
6.将此选区保存为alpha通道，更名为masked
7.在masked这个通道激活的情况下，复制到新的层里(运行“通过拷贝的图层”或者CTRL-J)
8.在这个新层下运行高斯模糊(半径14)，是比较高的一个值，你可以多调几次，直到得到满意效果为止。一般情况下，越大的图片，或者越大面积的皮肤，需要用越高的高斯模糊值。
9.将刚刚模糊过的图层的不透明性设到70%，看看是不是得到了让你满意的效果(要避免太高的模糊值，以免抹杀了那些皮肤的细节了)。最好的效果是能看到一点点皮肤的细节部份，比如毛孔，等等。
10.缩小图象到95像素，保存你的工作

现在的图片已经是缩小了的了，要进行USM锐化，以便使皮肤的细节部份更加明显

1. 我用USM锐化@ (25%，半径0.5，阙值0%) 4次，记得不要锐化过了！
2.下一步，我载入了"skin" 选区，继续进行锐化@ (1. 25%，半径 0.5，阙值 0% ；2. 25%，半径 0.3，阙值0%；3. 15%，半径 0.3，阙值 0% ；4. 8%，半径0.3，阙值 0%)
3.多进行几次低数值的USM锐化，所得出的效果比直接进行一次高数值的USM锐化要好。
4.选择头后面的背景，并建立到一个新层里，之后对它进行高斯模糊(为了减少由于锐化所产生的杂质，既那些小小的颗粒)
4.在原图 (就是最初的那个“背景”图层) 的锐化过程中，会发现第二个被高斯模糊过的层 (就是那个刚才新建的那个层) 很好的遮挡了那些锐化产生的杂质。
5.作为图例说明，下面的图是我的背景图层( 没有加上那个模糊过的皮肤图层)