If you're shooting in a very bright and low contrast situation, the pictures might look a bit washed out even if the exposure is technically OK. You might try exposure compensation (in the minus direction) to darken. Don't overdo it. I had this issue recently in Africa, where there is little contrast and the dynamic range of the actual scene is fairly low. How much you need will differ considerably from one scene to another, but if you start with one stop negative, you can see whether you're on the right track.
If you have Active D-lighting enabled, you should turn that off, as this tends to reduce overall contrast and lighten shadows. If you are bothered by shadows that are too dark, you're better off fixing it in post.
If you're shooting in Raw mode, you can use a Raw reading program like View NX-2 or Capture NX-D to do the exposure compensation in post, as well as opening up shadows when the need arises.
If your exposures are not showing blown highlights, and the problem is simply poor contrast in the scene, post-processing may be the best solution as negative exposure compensation in post can also reduce noise a little, but if you're shooting in JPG, you should compensate in camera.
If you are using a high shutter speed or a very small aperture and Auto ISO, the ISO might be going higher than is best, and that can reduce dynamic range also.
Make sure that the camera is not accidentally compensating already, and make sure too that if there is a lot of ambient sun, you use a lens hood to avoid flare, which can reduce contrast greatly. UV and other protective filters may make flare worse, as well, but you can sometimes get a little boost in contrast and saturation with a circular polarizing filter.
In the replay menu, you can choose various display options, one of which is called "overview." This will give you some basic information on the settings of a shot you've made, and will also show a histogram of your shot. If the information on the histogram is crowded toward the right, and hits the right margin of the graph, this is a sign that you're overexposing. Try reducing exposure until the information falls more or less between the two margins. Do not worry about the height of the information, as this depends entirely on what colors and shades are in the scene.
You can also enable the "highlights" view, which will cause blown highlights to blink. If more than a few little specular highlights blink, that's also a sign of overexposure.
Once you've enabled the various options, you can choose them with the up/down arrows of the back control when you replay.
I would not add more sharpening for this particular problem, unless your images are also unsharp. However, many programs allow you to adjust contrast either for Raw or JPG images, though at some cost in shadow detail, and a simple boost in contrast might be a good start. A simple change of brightness might also work. Depending on the program many possibilities may present themselves including a change in levels, which actually alters the right and left edges of the histogram, at the expense of dynamic range, and a change in "gamma," which has similar effects to a change in contrast.
I'm assuming in all this that you're using P,S,A or M mode in your exposures. Other modes will reduce your ability to control things. If you're not comfortable making your own shutter and aperture adjustments, use P mode, which still automates those, but leaves you all the options for focus mode, ISO and the like.
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If you have Active D-lighting enabled, you should turn that off, as this tends to reduce overall contrast and lighten shadows. If you are bothered by shadows that are too dark, you're better off fixing it in post.
If you're shooting in Raw mode, you can use a Raw reading program like View NX-2 or Capture NX-D to do the exposure compensation in post, as well as opening up shadows when the need arises.
If your exposures are not showing blown highlights, and the problem is simply poor contrast in the scene, post-processing may be the best solution as negative exposure compensation in post can also reduce noise a little, but if you're shooting in JPG, you should compensate in camera.
If you are using a high shutter speed or a very small aperture and Auto ISO, the ISO might be going higher than is best, and that can reduce dynamic range also.
Make sure that the camera is not accidentally compensating already, and make sure too that if there is a lot of ambient sun, you use a lens hood to avoid flare, which can reduce contrast greatly. UV and other protective filters may make flare worse, as well, but you can sometimes get a little boost in contrast and saturation with a circular polarizing filter.
In the replay menu, you can choose various display options, one of which is called "overview." This will give you some basic information on the settings of a shot you've made, and will also show a histogram of your shot. If the information on the histogram is crowded toward the right, and hits the right margin of the graph, this is a sign that you're overexposing. Try reducing exposure until the information falls more or less between the two margins. Do not worry about the height of the information, as this depends entirely on what colors and shades are in the scene.
You can also enable the "highlights" view, which will cause blown highlights to blink. If more than a few little specular highlights blink, that's also a sign of overexposure.
Once you've enabled the various options, you can choose them with the up/down arrows of the back control when you replay.
I would not add more sharpening for this particular problem, unless your images are also unsharp. However, many programs allow you to adjust contrast either for Raw or JPG images, though at some cost in shadow detail, and a simple boost in contrast might be a good start. A simple change of brightness might also work. Depending on the program many possibilities may present themselves including a change in levels, which actually alters the right and left edges of the histogram, at the expense of dynamic range, and a change in "gamma," which has similar effects to a change in contrast.
I'm assuming in all this that you're using P,S,A or M mode in your exposures. Other modes will reduce your ability to control things. If you're not comfortable making your own shutter and aperture adjustments, use P mode, which still automates those, but leaves you all the options for focus mode, ISO and the like.