I know when I first got photoshop I was so excited to use it that many times I was guilty of using it a bit too heavily. While a lot of effects look new and awesome they don't always fit well with every picture. In many cases we want to preserve realism in our art, that is when you have to be careful. Photoshop is a beautiful tool when it comes to enhancing art, sometimes you just have to know what to use with care.
1. Dodge and burn.
When you use the tools dodge and burn you can create amazing effects BUT if you want to keep your picture as natural as possible it's important to remember a few things. These tools can be very destructive, both to pixels and realism. And if you over do it you will very easily end up with messy blown highlights and dark unattractive patches. Light and shadows have a very special interaction, and if you don't follow how they fall, move and interact you can't achieve a natural result. When dodging be very very careful with the "highlight" setting as it tends to "burn" the light into the picture. If you use it on a very low setting it can give new life to areas that need it, but go too far and it will end up looking very bad. The same thing goes with the shadows, while I do find them easier to handle. Low settings are the key, you can add more effects with time.
We all love color and life! But there's such a thing as "too much". I was very guilty of doing this a lot before, and while there is times this over the top look actually fits, for most pictures it does not. Adding some saturation can often pop extra life into the picture but do not add too much! Much rather experiment with color balance and lighting. Instead of using ONE effect too much, try subtle uses of multiple ones. Something I do with most "every day" kind of pictures is that I just slightly raise the saturation as well as experiment with levels. More often than not, this is all that is needed.
3. Overdone HDR photography
HDR can look amazing, it's incredible how much life it can blow into a picture. But there's few things that can look as ugly as a badly made HDR picture. We all see them everywhere, the HDR pictures that are so overdone that it doesn't even resemble real life anymore. When using this with care, it can give amazing results! And when seeing that it's easy to want more and take it way too far. Always add back a percentage of your original picture and if you're unsure if the effect is too strong or not, lower it and experiment a bit in photoshop.
4. Over sharpening
Almost as bad as the overdone HDR pictures....
Over sharpening never looks pleasing, it does not flatter the picture, not portraits, not landscapes, nothing. It gives a very unprofessional look and it's not something your eyes will want to rest themselves on. When sharpening do it with CARE, do it little by little and do it in a way that gives you control. I prefer using highpass sharpening or unsharp mask over the normal "sharpen" that gives no control at all. I try different settings, often mix them together, and I use them all in a subtle way.
5. Forgetting to save your work
The biggest danger of being so excited to work with your pictures is that you forget to save them often. I have lost days work this way and it's no fun. Make it a habit to save every five or ten minutes and you won't use any precious work.
6. Sticking to the familiar
When using photoshop it's only natural to find a work flow that works for you and your art. Keep it! But also don't be afraid to try something new or you might miss out on some amazing things. Photoshop is filled with opportunity, you must explore and discover to find even just a fractal of all of it. Even if it's just trying a slightly different setting, it can prove itself to work wonder on that special image you have.
7. Going with too strong of a contrast.
Don't get me wrong, this look can turn out amazing if you know how to handle it and have the right kind of picture of it. But chances are that you apply it too strong and too carefree to a picture that it does just not flatter. The more you raise the contrast, the more you're risking overblown lights and detail less shadows.
8. Taking the simple way.
For exemple going straight to the contrast bar instead of using levels that will give you TONS more control. When using levels you can control both the midtones, highlights and shadows, by simply adding contrast you can't. If you are aiming for a contrasty look, levels is the way to go as well. You must have control with this or your picture will most likely end up looking a mess. This goes for most things in photoshop, simply adding auto contrast or auto color is quick but in many times not very effective. Just like adding a quick sharpen will leave you with very little control. Take that extra step and your image will look a whole lot better.
With all of this said, of course you should experiment and make some errors, just be a bit cautious with taking it TOO far at times!