Photoshop shortcuts can be incredibly time-saving as you will know, however, there are some that I didn't know of until many years after I first started using it. This list of Hidden Photoshop Shortcuts will help you become a more efficient Photoshop user and save great big chunks of your time when working on your next project.
1. Ctrl/Cmd + Enter
When using text, instead of clicking the arrow key on the tool menu to apply the text, simply press Ctrl/Cmd + Enter when you're in the text box.
2. Numbers (opacity)
Use the numbers on your keyboard to change the opacity of an object. 1 = 10%, 5 = 50%, and so on.
3. Ctrl/Cmd + Alt + Shift + E
To flatten layers without losing them, open a new empty layer and press Ctrl/Cmd + Alt + Shift + E. This is an incredibly useful one!
4. Ctrl/Cmd + 0
Fit to window. This immediately scales the canvas to fit as large as possible in the window.
5. Ctrl/Cmd + Click
To select layers quickly, Ctrl/Cmd + click will select an object and its layer. Alternatively, Ctrl/Cmd + drag to create a box that selects multiple objects and its layers.
6. Alt (in brush mode)
Press alt when in brush mode to switch to the eyedropper tool, in order to sample a colour from your image.
For a larger workspace, press tab to hide/show menu and layers.
8. Shift + (shortcut letter)
To quickly toggle through tools that have more than one tool palette position, press the shift + (shortcut letter) until the desired tool is selected.
9. Hold Alt/Option + Click
To hide all other layers apart from one, hold Alt/Option + click on the visibility icon (the little eye next to the layer name, in the layers palette.
10. Drag to hide/unhide
Drag your mouse over the visibility icons you want to hide/unhide. (Much quicker than clicking each icon!)
11. Ctrl/Cmd + Tab
Ctrl/Cmd + Tab to quickly switch between documents windows.
12. Alt/Option + Shift + Drag (when transforming)
To scale proportionally from the centre of an object, hold Alt/Option + Shift + Drag when using the free transform or marquee tool.
Hopefully you have learned some new shortcuts from this list. Have any others you can share? I'd love to put some more into use!