Adding a little contrast is often a good thing, but adding contrast is bad for certain parts of an image.  To add contrast in Photoshop Elements, do the following

• Enhance > Adjust Brightness/Contrast > Brightness/Contrast
• Move the Contrast slider to the right a little, usually in the +5 to +15 range.

Increasing contrast will generally make darks darker, lights lighter, and it will also add some color saturation.  These are generally good things for a photo taken on a cloudy day or on a shot entirely in shade.  The image can be enhanced in a wonderful way.  You generally want to avoid adding much contrast in real dark areas, real light areas, sky with clouds, areas with digital noise (which usually includes skies), and the faces of certain people.  The reasons are that the dark areas will become almost black, the light areas will get washed out to near white, clouds will look poor, and digital noise will increase to an unacceptable level.  Too much contrast on a face can accentuate wrinkles, freckles, and the resulting color saturation can be so great that they can sometimes appear to have a red rash on their face.

Be selective where you add contrast.  Make a selection, feather the selection, and then add a little contrast.  Zoom in and look at it carefully for unwanted artifacts.  A great place to add contrast is to a persons eyes, and possibly their lips, but not to the rest of the face.  By selectively adding contrast to the eyes and lips, these areas will stand out more.  Additional contrast is great on midtones of many images in that it accentuates the details and makes the colors richer.  Be careful about over doing the contrast above the 15 level because unwanted artifacts generally occur when more contrast is added.

I used contrast selectively in the examples given in the Selective Examples Page and the Layers Page.  In those examples, I increased the contrast only to the reddish central region below, but not to the sky or the white rocks in the lower left.  Those example includes some changes to the sky and the white rocks, but contrast was increased only to the reddish central region.
 
 
 

Fig. 1.  Before

Fig. 2.  After all of the changes in the Selective Example Page, which includes more than contrast.