If you want to know what to look out for when hiring a professional then please see the bad example below. Alternatively, if you are just starting out in photography and are experimenting with photoshop please, please avoid over brightening of eyes and smoothing skin out so much that it looks plastic, I shall post a bad example of editing below:
The main points that I want to touch on in the edited image above are:
Eyes: a common mistake when editing the eyes is when they are brightened so much that the whites are glowing and the entire eye has been so over sharpened that they look stuck on. If you are trying to make the eyes stand out a little more it is possible to do so without making them look fake, you just have to find the right balance. Remember they need to look a part of the image and not unrealistic compared to the amount of light that is actually lighting the face. =]
Skin: I have used color efex's dynamic skin filter at full effect to get the skin like this and even then it's not as plastic as what some images out there are, but basically, skin needs to look like skin, people do have wrinkles, spots, freckles etc, and they can be edited subtly or even got rid of completely and still look good its all about experimentation and not forgetting that your lighting needs to be spot on to get the shadows and highlights where you want them. There is no point in creating a lighting effect that you are later going to mess up completely by editing the face to be all one colour with no depth because you have over photoshopped it. ( I know I ramble abit but stick with me! )
Teeth: Yes, it's great to have white teeth but over brightening them so they turn into a rectangle of glaring white is not going to do your image any justice, it can be difficult to edit teeth when they are yellow or wonky but the best method I find is using the lasso or select tool to select all the teeth and with a big brush whilst set on dodge mode do a quick sweep over the whole area but make sure your opacity is set low to avoid over brightening. If the teeth are yellow desaturate them first with the sponge tool making them grey and then lighten with the dodge tool that way there will be no yellow tinge. This takes practice but is well worth it once you get the hang of it.
One of the most common mistakes in photographs that I find is they are overexposed and the photographer seems to have used it within their editing. It does, to some extent, come down to personal preference but there is a difference between high key images, where you set the lights up to give you that extra light and being overexposed, where the highlights are so blown out that there is no detail in them and you squint when looking at the image.
Here are some examples again edited to show the difference:
Balancing exposure can be difficult especially when shooting with lights and then having a window/curtain lit from behind but if you experiment, use a light meter and always make sure your model is exposed properly you should be fine.
I hope this post has been informative and enjoyable for you to read. Please feel free to comment or email me if you have any questions or would like some advice on your portfolio.