My basic approach to sharpening was influenced by the late Bruce Fraser, particularly this article that he wrote so much so that the general approach, not just sharpening images, extended to my overall post-production process.
Warning: this is where it get’s geeky 
So basically, what I’m looking do with an image to get it sharp is to:
- Capture: make sure the converted RAW file is has some sharpening applied to it. I use Capture One and I find it does a fantastic job of this with it’s conversions, unlike Lightroom or ACR where I have to apply a sharpening setting to the image to get it where I want it.
- Creative: apply sharpening to the image during the creative processing. This usually in the form of targeted unsharp mask to the certain areas of the images (eyes, lips, fabric) and the use of the high-pass filter after low pass processing (aka using some kind of blur or inverted high-pass. If you really want to get your geek on and truly understand what I just wrote, go here and bask in the glow of tech talk.
- Output: I don’t apply overall sharpening to an image until I know what the final output is for the image. Most of the time it’s for the web so I apply the smart sharpen filter to my 750×500 pixel image, usually setting of Amount:100%, Radius: 0.5 px, Remove: Lens Blur. You will want to play with the Radius slider to salt to taste for another size image. I find that you want to be as aggressive as possible before you start to see contrast halos.
