Deformation, polygroups and clip brushes

 Deformation:

The Tool > Deformation sub-palette provides ways to sculpt 3D objects using unique effects. To see these effects updated in an object on the canvas, it must be in a Transform or Edit mode.

Base model

This is the same model. I applied a mask around the eyes and the cheeks to make sure the deformers won't change them. I then applied Spherize, Gravity, Inflate, Taper and noise to create this deformed looking version.

Polygroups:

Polygroups allow you to organize the mesh with visual grouping information. Polygroups are one way to organize your mesh.

Before adding polygroups over the base model.

After adding polygroups onto the model. To do this, I highlight the area using CTRL to create a mask, then while still holding down CTRL, I press the W key to turn it into a group. I do this for each area forming multiple selection groups. 

Press W to bring up the modifier menu, the CTRL + Shift to select an area that you have applied a group too. You can then use the tool to move and altar just the area you have already highlighted and grouped. 

Clip Brushes:

The Clip Brushes are different from any other brushes as their operation is always perpendicular to the canvas. These brushes are similar to how the Eraser tool cuts away pixels on the canvas, except that they are used to cut away areas of your model and slice its borders.

This is my model before I use a clip brush to create flat cut off layers.

Here I have used clip brushes in order to cut sections off the model creating a smooth and flat cut cross section. I use the Trim Curve brush Shift + CTRL to bring up the brush/cut line for where I want the model to cut off. I line up the area and make sure the brush is going to chop away the right side of the line on my model. If it isn't I can use the ALT key to flip it. Once happy, you let the keys go and the section will be cut to the line you highlighted across the model.