So I spend a lot of time fixing these flyaways, and I’ve learned a thing or two about the process. There are lots of approaches that work, but only a few that work consistently, and really well, without driving the retoucher up a wall. Equally important is a technique that doesn’t require that you spend all afternoon working on the hair, or that you rebuild the photo and background from the ground up.
With the clone stamp selected, and the mode set to Normal, reduce the opacity to 50%. Make sure the brush you’re using is set to a hardness of 0%, and that it’s big enough to blend (i.e. feather) nicely with the background, but not so big that it will affect the bulk of the hair. (As you get closer to the subject’s head, you may have to make the brush size smaller to ensure you can make the fine-tuning adjustments as necessary. But, until then, use as big a brush as you comfortably can.)
In some instances, experimenting with the stamp mode set to Lighten or Darken can mostly alleviate stray hairs. The problem with the Lighten/Darken approach is that it almost always leaves a ghost image of the hair—which can be just as distracting as the original hair was in the first place. But if you’d like to start this process with the Lighten/Darken approach, by all means feel free. (For bright hair on a dark background, the Darken setting will eliminate the glowing hair, leaving usually just a faint dark outline. In the opposite instance, a dark hair on a light background, choose the Lighten setting.) Just know that you’ll have to come back and follow up with the Normal clone stamp set to a low opacity to finalize the edit. Or you can skip the Lighten/Darken stamp altogether, because the next step works so well you might as well save the trouble.
With the clone stamp set to Normal and the opacity somewhere below 50%, start clicking and cloning to replace the distracting hair with tones from the background. The real key, if you have the patience, is to set the opacity as low as you can comfortably go—perhaps only 20%—and make several clicking brush strokes to slowly, subtly, paint away the hair.
The success rate usually correlates to how accurately you choose a portion of the background that corresponds nicely to the area you’re eliminating. The nice part about this technique is that even if you choose a background area that's too light or too dark, because you’re making low-opacity, subtle changes, your chances of overdoing it in one motion are next to nil.
For successfully retouching hair, remember that slow and steady wins the race. That’s where the patience comes in. Keep clicking and painting as you watch the flyaways slowly disappear. When in doubt, remember not to get too heavy handed. Our eyes and brains are smart enough to know that nobody’s hair is perfect. If you remove too many strays and create a perfectly smooth hair helmet, it’s bound to look just as bad—if not worse—than the original head of hair, flyaways and all. Take your time, and don’t go overboard. You’ll be amazed at the powerful results you can get from this simple approach.