For accurate masking it's important to know how well the image will look on either a dark or white background.

Add two layers underneath the image that has a mask and fill one with black and the other one with white. Mask your image and turn on either the layer with black or the layer with white to check how accurate your mask is and to avoid ugly edges (not smooth, wrong color) around your extracted image.

To hide all layers except the active layer hold down the Alt key (option key on the Mac) before you click on the eye icon in front of your active layer. Click on the icon again to return to the previous state.

The Save for Web (File / Save for Web...) window offers a quick preview of the image in a web browser; you need to click on the browser icon at the bottom of your preview window:

I you want to try out different blending modes in your layers palette, then click twice inside the blending mode box to activate it...

..and then use the cursor up/down keys on your keyboard to preview the effect of the other blending modes.

With the layer active that needs to be copied, press Ctrl + J (Command + J on the Mac).
To be able to give the layer a new name at the same time: press Alt + Ctrl + J (Option + Command + J on the Mac).

If you have a brush selected and you click on the Brush Preset picker button (A), followed by clicking on the fly-out menu (B) then you'll notice that there are several brush collections available that you can use (C):

How can you add more collections, for example the ones downloaded from the internet?
You can do that by copying the brush collection (a file ending with .abr) to the following folder:

C:\Program Files\Adobe\Photoshop CS\Presets\Brushes

Restart Photoshop in case Photoshop was already running when you copied your collection or the collection won't show up in the collection list (C). Note: In my example you have to replace the section named Photoshop CS with the name of your version of Photoshop.

I don't have a Mac, so I appreciate it if someone who does own a Mac can tell me what the correct location is on the Mac

When your brush tool is active, right click on your image (Control + click on the Mac) and the Brush Picker window will pop up:

You can repeat the last filter by pressing Ctrl + F (command + F on the Mac)
If the filter had options, then you can press Alt + Ctrl + F (option + command + F on the Mac) to open the dialog box of the last used filter.

Immediately after applying a filter, you can decrease the amount of the effect applied by pressing Shift + Ctrl + F (shift + command + F on the Mac).
You can also select in the menu: Edit / Fade xxxxxx .... where 'xxxxxx' is the name of the effect you used last.

Sometimes you want to return to the default settings of a tool. Instead of clicking on the cancel button, hold down the Altkey (option key on the Mac) to change the cancel button into a reset button: