Liquid Resists
You apply a liquid resist to dry paper.  When it dries, you apply your paint.  When the paint dries, you remove the resist, usually with an eraser-like tool called a heel.  In the example above, you see the white of the paper, but at this stage you can also paint in the trees any way you want.
Liquid resists are very convenient for small areas.  It helps to dip the brush  in soapy water first, since liquid resists can be hard to get out of a brush.  Make sure you rinse the brush in soapy water right away after using.  If the resist is allowed to dry in the brush, it will ruin the brush.  For this reason, it is best to not use expensive brushes for this application.  

Since a liquid resist slightly affects the surface of the watercolor paper when removed, it is better not to use this kind of resist for a large area.
  

Opaque White
Sometimes Chinese White is a good option  if you need to paint something lighter in an area where you've already applied a darker paint.  
It is better to mix Chinese White in a separate dish, because it is chalky and can dull your transparent colors.

You can also add colors to Chinese White as needed.




 

Tape
Artists Tape (or masking tape in a pinch) can be used to tape off areas before you apply paint.   You might want to use a spoon to burnish the tape edge to prevent any paint seepage.  Apply the tape onto dry paper, and  remove only after the paint application has dried. 

Wax resist
You can use wax crayons or wax candles for a permanent resist that gives some texture.   Remember that once you apply this resist, it is permanent and will not come off.