Photoshop is full of keyboard shortcuts. Many of them are hidden from the user. Here are some of the most useful shortcuts to learn.

Working with tools

Larger/smaller brushCtrl + Option + drag left/rightRight Mouse + Alt + drag left/right
or left/right brackets [ and ]
Harder/softer brushCtrl + Option + drag up/downRight Mouse + Alt + drag up/down
Temporarily access Move tool with other tool activeCommandCtrl
Temporarily use Dodge when using Burn toolOptionAlt
Move, Marquee, Lasso, Pen toolsV M L P
Brush, Stamp, Burn, Healing toolsB S O J
Change brush strength to 10%, 20%...100%Number keys 1, 2 … 0
Change brush strength to eg 35%Number keys 3 then 5
Temporarily access Polygonal mode with Lasso toolOptionAlt
Watch a 2-minute video on Brush tool essentials

Working with selections

Before drawing selection:
Add to previous selectionShift
Subtract from previous selectionOptionAlt
Intersect with previous selectionShift + OptionShift + Alt
While drawing selection:
Constrain to square/circleShift
Draw from centre outOptionAlt
Move selection while drawingSpacebar
Watch a 2-minute video on working with selections

Viewing channels

View red/green/blue channelCommand + 3/4/5Ctrl + 3/4/5
View RGB channelCommand + 2Ctrl + 2

Working with layers

Move layer up/downCommand + [ ]Ctrl + [ ]
Select layer up/downOption + [ ]Alt + [ ]
Nudge layer 1 pixel (with Move tool) Up/down/left/right arrows
Nudge layer 10 pixels (with Move tool) Shift + up/down/left/right arrows
Cycle through layer modesShift + plus / minus keys
Fill with foreground colorOption + BackspaceAlt + Backspace
…preserving transparencyShift + Option + BackspaceShift + Alt + Backspace
Fill with background colorCommand + Backspace Ctrl + Backspace
…preserving transparencyShift + Command + Backspace Shift + Ctrl + Backspace
Open Fill dialogShift + Backspace
Set colors to default black/whiteD
Swap foreground and background colorsX
Lock layer transparency/
Enter/Exit QuickMaskQ
Copy MergedCommand + Shift + CCtrl + Shift + C
Change layer opacity to 10%, 20% … 100%1, 2 … 0
Change layer opacity to 0%0 0
Change layer opacity to eg 35%3 then 5

Working with dialogs

Load with previous settingsOption + menuAlt + menu
Increase/decrease valuesUp/down arrows
Increase/decrease values x10Shift + up/down arrows
Toggle Preview on/offP