gui, the way i do it is utlizing the "color exclusion sliders".

you know when you have the "Layer Options" dialog box up? There's a number of options available (i think 7 in all but it may differ between versions...)

if you look at the "Blend If" option you'll see it set to "gray" (this is the default and means any change you make affects ALL color channels equally.

- on an ODD note this is one time GRAY means ALL COLOR CHANNELS!! all other times in the app GRAY means GRAY COMPOSITE...)

:)
anyway,
By selecting a color channel from the "Blend If" menu will apply the effect of the "slider bars" to the one channel.

EXAMPLE:
The first slider bar, "This Layer" hides pixels in the active layer according to the brightness value (you can lose the dark pixels by dragging the LEFT slider/triangle or toss the light pixels by dragging the RIGHT slider/triangle)
The second bar, "Underlying" will force the pixels in the underlying layers to show through according to the brightness value (to make the dark pixels show on the underlying image, drag the LEFT slider/triangle and for the light pixels, drag the RIGHT slider/triangle)

What i like about this is ALL the adjustments done in the "Layer Options" dialog box are NOT destructive (meaning the pixels are only hidden NOT deleted so you can restore pixels or hide new ones!!)

There is a DOWNSIDE to this...
Hiding and Forcing colors can make some harsh color transitions The trick is to soften the transitions leaving some pixels opaque and others transparent...

NOW FOR THE TRICK:
To do this you need to "OPTION/ALT +drag" on the sliders. This splits the triangle into two halves, the LEFT represents the beginning of the "fuzziness" (brightness value) and of course the RIGHT represents the end.

Play around with a couple of images on different layers and see what you can come up with, then post it and share the experience!

:)
s