After cooling stock or a dish in the refrigerator overnight, the fat will congeal on the top of the liquid.  When making stock or soup, this is a great time to remove the most fat.  Simply scoop out the now opaque fat and discard!

Another helpful hint is to use cheese cloth or a paper towel depressed a little bit into the soup or stock a bit when first placed in the refrigerator.  The next day merely pull the cloth or towel out and the solid fat will come with it.

Another option is to keep the fat to use for cooking later.  This is especially a good idea when removing duck fat or game bird fat.