Have you ever gone on a vacation, or even a day trip and taken some outdoor landscape images that looked great only to find that when you looked at them in your computer, they looked a little flat? Despite our best efforts of determining the best exposure, white balance, color balance, etc.. some images just don't look like we remember them. In some cases, it may be the dynamic range of the camera (ability to capture the full range of highlights, midtones, and shadows). However, even the most expensive cameras can not capture a full range in every lighting situation.
There are quite a number of third party plug-ins to photo editing programs that can improve outdoor images such as Color Effects Pro, Perfect effects, and many more. These plug-ins can be helpful if you are tight on time and need to get quality images out quickly or even if you just want to see a variety of looks for your images. I believe though it is always a good idea to at least learn some of the basic tools, just in case you don't have these third party plug-ins or just want to have ultimate control over your images. The getting the sky just right with trees or other subjects can be tricky.
If you have lightroom, you can start your image editing there. Steps include:
Not every image will need each of these adjustments, and yet some images may need to imported into photoshop. Here is a very tutorial that is quick and easy.