Good tutorial. A cool 'levels' trick that I learned is:
When you move the black/ white point sliders, you are clipping/ saturating some pixels. Now sometimes you want this (like in your version here) but in general it is pretty hard to tell this is happening just watching the preview.
However, if, when you move the slider, instead of just clicking and dragging it, you press Option/ Alt and then click/ drag, the preview will show you the pixels that are being clipped (and which channels they are in for a colour version) so that you can really precisely control how much you are clipping, and where.
Try it - press alt (on a PC) and drag the sliders around, the preview will go all white or black (depending on which slider) and then slowly bring in the highlight/ lowlights that you are saturating as you move the slider.
Helps a lot to get the full range without screwing up too much, though I tend to prefer using the curves command to mess with contrast, just for the increased control.
Photoshop is a bit like that - far too many shortcuts and 'hidden' features, almost like they wanted to make money selling more books :)
Ctrl-Alt-Shift-E is a handy one to know too :) Collapse all visible layers into a new layer and make it visible. Feels like you are about to reboot the computer though...
