As Louis P pointed out, one key thing is not to use a 100% brush. When I remove stray hairs, I use the Clone stamp in Airbrush mode, 100% opaque, 47% flow, and usually around 80% hardness depending on the background. I find it works better to make two passes with a 47% brush than to make one pass with a 100% brush: I often slightly change the sample point on the second pass *if* the background is soft and mostly mono-colored, like the wall of my studio (it's a sueded finish.) Obviously if the background has some sort of pattern, like a brick wall, you want to replicate it as faithfully as you can.
Following the curve of the head is also good advice if you can do it without breaking up the background pattern. Creative choice of sampling point is important: you may need several sampling points as you work your way around. When cleaning up an irregular hair profile, it can also sometimes be a good idea to sample some hair from further inside the curve and fill in the edges.