In the interest of not re-inventing the wheel, I’d like to point out that there are many marvelous websites with directions on how to thicken. The Wikihow sites  are quite instructive (you could learn to cook by following their step by step illustrations) and the comments afterward add even more tips. Do take a look:

http://www.wikihow.com/Thicken-Soup

http://www.wikihow.com/Thicken-Sauce

http://www.wikihow.com/Thicken-Gravy

http://www.wikihow.com/Thicken-Liquids

 And there are more: how to thicken stew, spaghetti  sauce, etc. And remember this: what is thickened can be thinned. In other words, if your recipe ends up thicker than you wish, add water, milk, cream, or broth to thin it.