Good job on the vehicle studies! And yeah, seconding Stefanie here: finding a balance between soft and hard brushes will make certain areas of your painting pop out more ( depending where you want the viewer to pay attention to), instead of relying on one brush mode all the time. Experiment with it! [:
As for portrait studies/ whatever (organic) study you are doing, what helps is looking at the negative space around the object you are drawing and try to mimic that shape in your painting for better accuracy. Try not to rely on what you already now but take knowledge in. By doing that there's a smaller chance of drawing 'icons', for example, literally drawing an almond shape and a circle to indicate an eye. Hope that my rambling helped a bit ^^''
