Every single picture that comes off your camera can benefit from sharpening. Sharpening is a process which helps to define individual pixels. If you compare an unsharpened photo to a sharp one, the unsharpened picture will look blurry. Sharp is good.
I purchased a tutorial from Erin Cobb last year that included how she sharpens images. She's a fantastic photographer and I was happy to learn her post-processing tips.
Yesterday in class we learned about the Smart Sharpen filter. This was new to me... Erin uses the Unsharp Mask and so did I. We learned yesterday that the Unsharp Mask is an older technology in Photoshop that is not as effective or accurate as Smart Sharpen.
You can find Smart Sharpen by going to Filter > Sharpen > Smart Sharpen. Once the dialogue box opens, it will look like this:
The Amount slider is like a volume control. You can turn up and down the sharpening. This is a pretty safe control and won't get you into a lot of trouble even at the higher ranges. I start with 150% and go from there.
The Radius slider is what can get you into some trouble. The radius refers to the width of the 'halo' that goes around the pixel. This should really never go above 2... maybe 3. I think the best way to check is to start at 1 and then move up if you need it. Your eye is your best judge. Photoshop allows you to add a ridiculous amount of radius, but Lightroom won't even let you go past 3 (I'm pretty sure) because you don't need it. (Remember, Photoshop was created for Graphic Designers; not all of the functions are good for using on photographs!)
I should mention that when sharpening in Photoshop, you are applying the sharpening to the whole photo (the color information) and not just the luminance. If you are sharpening in Camera Raw, it works in reverse and only applies the sharpening to the luminance, which tends to keep the image looking more realistic and less overly-sharp.
Another sharpening technique that I learned is in Lightroom and Camera Raw. If you use the clarity slider, you are actually sharpening the midtones of the photograph. A good range to move this slider is between 20-25.