Here are a few basic tips and tricks for working in Photoshop. I intend to expand this section with more information when I get time.

Load a selection from a layer

Ctrl-Click a layer to select its non transparent area

Drag-Duplicate a layer/selection

Hold Alt and start dragging. If you drag a selection it will be duplicated within the layout, if not it will duplicate the whole layer.
Holding Shift in addition will make it drag strictly horizontally/vertically.

Use masks

Create a mask from a selection by clicking the mask button.
You can assign masks to groups and art layers. You can also create vector masks with vector shapes.

Using Curves

3 points: shadows, highlights, midtones

point on image to see the location of that pixel on the curve
hold Ctrl to create a point on the curve

Cutting Levels

Cut off empty areas. Beware! Doing this can lose you a lot of image information. Further modifications may result in ugly colour/brightness steps. That’s why it should be done right at the end.
It is better to use curves where possible.

Drawing vector paths

– point, next point, drag, alt-click point, next point, drag, alt-click …
– set your points at the extremes of a shape
– set your points at vertical/horizontal tangents

Essential keyboard shortcuts for artists:

Space – pan work area
B – Brush
E – Erase
R – Blur/Smudge/Sharpen tool (can be handy)
F – toggle fullscreen, hit multiple times for different modes
Tab – hide menus
Ctrl and +/- – Zoom
[] – Change brush size
0 through 9 – set brush opacity or any other selected value

up/down/left/right – move 1px
Shift + up/down/left/right – move 10px

Ctrl and G – Group layers

More to come:

> make use of patterns

> Create a dotted line
with a brush (trace a line)
with shapes (repeat transform command)

> Photoshop actions
batch actions
run a script

colour proofing