Sharpening is an Optical Illusion
Users often struggle with sharpening their images. Sharpen too much and your images will look unnatural and harsh. Not sharpening enough though will leave you with soft images that lack impact. Part of the problem with solving this is that sharpening isn't really possible. It's really just an optical illusion.
Pause for a moment and consider just what "sharp" means. Among its many definitions, the relevant ones all include the idea of "having a thin edge or a fine point." That makes sense. Clearly the best way to create a sharp image would be to capture one that way to begin with. Being careful to use your best technique with a lens having the best glass possible mounted to a camera with the highest resolution you can afford should do the trick, right?
Many of us have to make compromises in the gear we use to at least some degree, but even if you can afford the best there is, digital image capture inherently makes ultimate sharpness difficult to achieve in camera. Most camera sensors include an anti-aliasing filter that purposefully adds softness as a means to avoid nasty moiré pattern aberrations caused by the regular arrangement of photosites (pixels) on the sensor surface. Beyond that, the mere process of forcing reality through the cheese grater that is that defines array of photosites means that details only get captured at certain points. Even high resolution sensors don't capture continuous toned images. Anything between those points never even gets seen by the sensor and is thus not captured. Digital photography turns everything into series of discreet numbers.
The purpose of post-capture sharpening is to add back at least some of the details inevitably lost through digital capture. But where are those details to come from if they were never captured to begin with? When you look at it objectively, this whole sharpening thing just isn't really possible after the fact.
But we can create the appearance of sharpening nonetheless by accentuating contrast along edges and boundaries in an image. This works because the effect looks similar to what a truly sharp image looks like with its thin edges and fine points. The eye sees what it recognizes as sharpness and believes what it thinks it sees. The boundary appears to be better defined, so it looks sharper even when it technically isn't.
Let's consider a typical edge boundary in an image. For our example here, suppose one side of the boundary is light gray, and dark gray on the other side. I'm using shades of gray here rather than colors so that I can talk is terms of simple brightness rather than attempting to deal with how the human eye and brain perceive color, but the idea remains the same regardless. Capture such an image with complete sharpness and the boundary between these two shaded areas would be well defined and distinct, with one tone right up to the line, and the other beginning immediately on the other side. A softer version would be similar except that the edge would be fuzzier with a gradual falloff from one tone to the other in place of a crisp edge. The problem is that not every detail of the subject could be recorded accurately enough to do justice to the actual dividing line, and some of the resulting pixel values came out as averages rather than pure samples.
You could still visualize where the line should be by approximating the midpoint of the transition zone, but that line wouldn't be very sharp. The gray on the dark side would progressively get lighter as it admixed with the bleed over from the lighter gray side, just as the light gray side would get gradually darker as you approached that midpoint line due to the inclusion of some of the spillover light gray. Increasing contrast along that edge would affectively darken the too-light area of dark gray, and at the same time lighten the region of dark gray that came out too light, recreating the appearance of a more well defined boundary. We're not really adding any additional detail or improving the accuracy of the details we did capture. All we're really doing here is guessing that the blurry edge is supposed to be sharper and by how much it should be sharpened.
There are countless problems here. There's no way to tell the difference between sharp edges that came out too soft in an image and transitions that were never supposed to be sharp to begin with. The two would appear the same in an uncorrected image even if they should have been different had we had sufficient resolution and accuracy in the captured image. But adjustments that would "sharpen" one would likely "sharpen" both though only one required it. And any given amount of added contrast almost certainly wouldn't be optimal for all edges and transitions in a given image. And if you accentuate contrast too much along a given edge and you'll be left with an unnatural halo effect that would likely be worse than the problem it was intended to correct. And all of this is based on a guess as to where that edge should be and what sort of detail should lead up to it on both sides.
So if you're having difficulty sharpening images it's because there are no easy answers. The difference between users experienced with sharpening and those who aren't is that the former group knows that sharpening is no better than an optical illusion while the latter still believes they still just don't get it. The truth is, we're all in this together.
The solution to this is to recognize the nature of what we call "sharpening" and approach the task in a different way. Rather than searching for the magic values to enter into the Unsharp Mask dialog, look instead at the image you are sharpening and do what looks best. Since sharpening is an optical illusion, the task should be done while looking at the image, not just the numbers in the sharpening dialog. Pay attention for when you've overdone it and back off some to avoid the halo problem. And recognize that different areas in an image will likely benefit from different amounts of sharpening and sharpening parameters since they didn't all start out as the same objects and edges to begin with. Depending on how you sharpen, it's not always possible to undo the result so take it easy. A little sharpening often goes a long way and will almost certainly give you better looking results than if you overdo it.
There are no one-size-fits-all answers for sharpening. Use your eyes and sharpen visually. Pay attention, go slowly, and do what looks right.