Whilst searching to improve a too sour brew I stumbled upon the "Barista Coffee Compass Diagram" by BaristaHustle.com. It is a very helpful tool that has a plethora of descriptions for bad tasting coffee, e.g. sour, bitter, dull, limp etc. based on these spots in the compass, you use the arrow on the right that points to the opposite direction of the word, to get back inside the center of the circle.
So for instance, when you have a sour brew (located at the left of the circle), try extracting more and using less coffee. These might seem like open doors, but they really do help you diagnose and improve your barista skills. Be sure to check it out to improve your next brew!