One of the reasons that images can look great on screen, but flat and lifeless in print, is the limited tonal range that can be produced. A print cannot have a brighter white than the paper (zero ink coverage), and the paper can only take so much ink before the receiving layer in the paper becomes saturated (more ink does not result in a darker tone. Instead, it results in a ruined print).
Since we have an inherent contrast limitation, we certainly want to make use of the contrast that we have available to us. As a general rule, a good print contains tones covering the full range between the darkest black that can be printed on the paper and the whitest white that can be achieved.
You’ve edited your image, and it looks pretty good. It contains (a very small amount of) black, white (specular highlights, maybe), and the full range of tones in between. Maybe you’ve also applied a curves adjustment or used the contrast slider to expand your mid-tones, resulting in more apparent contrast too.
The trouble is, some areas of your image my still appear a little flat. If you increase the contrast further, it may have a negative affect on other areas of your image.
The solution? Local adjustments!
Adding a local contrast boost to parts of your image can give your image an overall appearance of having more contrast. This is because the eye focuses on small parts of a scene at a time, and our brain builds up an overall impression of our field of view.
Using curves and levels adjustment layers in Photoshop to perform local adjustments of contrast and tonal relationships allows us to give the impression of greater overall contrast, and also to draw attention to areas of our image that we want the viewer to notice. It will also result in a print with more visual impact!
The image below is a nice portrait of a beautiful subject, shot in great light. Basic edits and a black and white conversion have been performed in Lightroom, and it’s looking pretty good. If it were printed as-is, it would make a fairly nice print.
What if we were to use a levels adjustment layer to adjust the skin-tones and give more contrast to the hair?
The Result is an image which will have more impact when printed (and also has far more impact on screen):
Don’t overdo it!
While your image should contain a small amount of pure black and pure white, that doesn’t mean that each and every part of your image should. The goal is to edit for a print that has impact and is engaging, not for one that draws attention to itself for the wrong reasons. Also, while you’re busily performing local tonal adjustments, be mindful of the overall tonal relationships in your image.