I have learned very, very, very much at editing your images, you POTN mates. Some were edited out of any request from the OP, just for practicing and because I loved the image and wanted to play with. Other times I tried to do my best to reply usefully to a precise question. I always got very supportive response from you.
I thought of creating this topic to reward the community of what I have learned here. I gathered links to some of the edits and other stuff I have done with step by step instructions to redo the same. I will be adding links from time to time, so please come back once in a while if you like it, and have any use of this stuff.
BW conversion
the black channel of a CMYK conversion can make a very nice high key portrait, like HERE on a wonderful image from martinsjc in THIS thread. The good of the K channel for portraits is: there is no black on the skin, and there is a lot on the eyes, eyelashes and eyebrows. if you are not after a high key portrait, you still can use the K channel to blend it in an existing BW conversion to bring more depth.
Color correction with a channel mixer layer
I found very difficult to do color correction and white balance with levels and curves, and still end with nice looking, natural and clear colors. A channel mixer layer is a very powerful alternative, especially in the two following situations:
1/ When a strong color cast is affecting the entire image
Channel mixer is more respectful of original colors when correcting strong color cast,
like HERE in THIS thread, on an awesome self portrait from Garrinn
like HERE in THIS thread on fotojoem's son portrait done with a wrong WB settings. No details explanation in this edit though: just the channel mixer layer formula (nothing more was needed to correct this great image 
)
2/ When the white balance is okay but skin tones need correction, Then the channel mixing has no competitor: if all your formulas have a total of 100%, you have absolute guaranty that your neutrals remain neutral while you work on the color tones.
like HERE in THIS thread, on a moody and delicate senior portrait from stepatee.
HERE in THIS thread from Geo, editing was challenging since Geo had already posted an excellent edited image. From the original, I firstly corrected the WB with a curve (the color cast was not strong, so a curve was okay). Then, once the neutrals were okay, I used a channel mixer layer to work on skin tones. I reproduced the same skin tones as Geo did with more classical tools. With the channel mixing method, the image gained in vibrancy, and clarity.
How to smooth skin and still keep the freckles
HERE (with blend-if technique) and HERE (with masking technique) on THIS thread by MikeZip007
More technical Photoshop stuff
On THIS thread, I demonstrate why a curve adjustment will change the saturation of your image. Such a side effect may or may be not welcome. I explain how to get control on it.
On THIS thread, I demonstrate how USM sharpness filter is changing colors in an undesired way, and how to limit this side effect.
Humor
don't expect learning anything useful here, here, here, here, here, here, here , here, and definitely here in this hilarious thread
Bottom note: I hope all these links are welcome by their original posters. Most of the time I have informed the OP ang got granted the permission to link here. If any of you wish to the link withdrawn, PM me and I'll do it. In all cases, Thanks very very much everyone for allowing me to post my edits of your images 
