Sometimes it's quicker by hand...

Satori provides the user with some very sophisticated methods of masking, but sometimes it's preferable to just dive straight in and paint the mask by hand.

Satori makes this job so much easier with the Brush Undo (Ctrl + Z) feature, which allows you to "wind back" the brush stroke. It's one of the coolest and most useful features of the program.

The method I use involves creating a copy of the image to be masked, creating a dummy filled colour layer beneath it, and then painting into the mask channel of my picture to reveal the dummy layer. This allows me to double check for accuracy by turning layers on and off, which is simpler and quicker than turning the mask on and off all the time. Also I don't have to use a visible mask, which I find distracting.  Take your pick, there are lots of ways to approach this job.

So, load up and get ready to mask

First, load the picture you want to mask. (You could right click on the image below and use "Save Image As..." if you want to follow this tip using the sample picture.)

This image needs to be shown against a transparent background.

Now copy the layer, and insert a new layer underneath it, as below...

The Copy Layer button(above) and the newly copied layer (right)

Add a third layer and then drag it down underneath the copied layer.

Fill the new layer with a flat colour. I normally use the lurid fuschia found just beneath the colour cubes, because this colour is rarely found in any of the pictures that I deal with, so it makes a good contrast.

Use the Filled Rectangle tool in Geometry to colour layer 3 with some lurid hue or other.

Click in the mask thumbnail of the top layer, and the Actions Palette will jump to Mask mode.

Make sure the top layer is active, and start to paint the mask with a Solid brush. Go round the outside of the shape using a small sized brush and fill in the rest with a large brush.
As you paint, you will reveal the flat colour layer underneath.  
I prefer to cut INTO the object to be masked, rather than cut out.

Avoid the temptation to flip the mask by using the "Invert" button in the Layer> Mask tab. This will add more objects to the stack and increase rendering time.
Stick to the Mask and Unmask buttons found at the lower left of the Actions Palette.

The middle layer revealed.

Don't forget the "Painter's Friend"

It should look like this when you've finished.

TIP # 1 You could use Geometry tools, such as the Closed Bezier Spline to make the mask for a simple object like this.
TIP # 2 As a final check, try re-colouring the middle layer with black or yellow to see if there are any stray pixels you have missed with the mask.

When you are happy with the mask delete the two bottom layers like so...

These 2 layers are now obsolete.

And you should end up with this...

In case you were wondering, it's an ancient coin from a recent illustration project.

You could use the tools found in the Keying tab of the Actions Palette, but more often than not you end up with stray unwanted pixels around the edges, and you wind up masking them out by hand anyway.