Steiner (STM1993 Version 3.00 beta)

[Ariel]

Ctrl+alt+z
Ctrl+shift+z
Those 2 will get u back and froth in ur history for longer mistakes

If u select a specific layer or use the selection tool on specific things u can go to Edit and will have the option to flip around there

Few more:
B - brush
Hold alt when using brush will get u the color picker
E - eraser
L - lasso tool
Ctrl + J - duplicate layer
Ctrl + T - transform
When using brush this '[' and ']' will change brush size
Ctrl + A - select all
Ctrl + U - hue/saturation changer
Ctrl + shift + v - paste on the same location
Ctrl + shift + X - modifying window thingy

Now to keep things not lose quality
When u change the whole img size
Under image > image size
U will see there at the bottom something the choose quality thing
I mainly use Billiner and Nearest neighbor
Those are the first 2

It will act the same when u re size a specific layer or selection
After clicking Ctrl + T for the transform tool
U will have that same choosing part for quality in the tool bar at the top of the window