While telling you to practice is the obvious and best advice, you also need to retrain your brain in how it sees things. You look at objects differently when you are drawing. Pay attention to the way light plays across the object's surfaces. How does it reflect light? There is reflected light even in shadows. What colors are in the shadows? How does the light and color intensity change? Before you start drawing just take some time and really look at what is in front of you. You will be really surprised at what you never noticed before.