Right Mouse Button + Shift
Add to selection: Ctrl
Subtract from selection: Alt
To add the next occurrence of the current word to the selection, use Quick Add Next, which is bound to Ctrl+D
Again, if you go too far, use Undo Selection (Ctrl+U, or Command+U on OS X) to step backwards.
To add all occurrences of the current word to the selection, use Find All: Alt+F3 on Windows and Linux, or Ctrl+Command+G on OS X.