PAINTING
The act of painting or laying down brushstrokes
1. When laying down general brushstrokes, do your best to not be neat. Work with a brush that has its opacity setting set less than 100%. By working this way at the start of a painting, (when you're goal is to be as loose as possible) you will create a noticeable overlap of color that gives you variations in paint density (OVERLAP-when a brush stroke passes over a previously recorded brushstroke on your image, this area has essentially been hit with an additive amount of paint that would turn an 50% wash of paint to a 100% wash of paint) (see tech. 1) This overlap will give you a nice representation of true paint effects. In the final rendering, these mistakes you make early on push the illusion that you painted this with real paint. The key is to not lose these overlaps or mistakes. Don't use the tricks that come with Photoshop, try to paint with a minimal amount of layers, and try to use a small amount of brushes, both styles and sizes. Don't use the lasso or path tools to make a perfectly selected area, allow the brush edge to define a shape. The unsteadiness of your hand and a given texture of a brush will give you the feel that you're working on a real textured surface. YOUR BIGGEST ENEMY IN DIGITAL WORK IS PERFECTION. It's easy to noodle a painting when you can zoom to 1/32 of an inch with resolution clarity, or when you have an unlimited amount of brush tips to work with. In reality, paint brushes are expensive, so you typically do not need every one that's in stock at your local art store, so treat Photoshop like reality and keep a limit on the amount of brushes you use. It will help keep a fresh feel to your work and eliminate any potential headaches caused by the overwhelming amount of resources Photoshop offers.

2. When you make the physical brushstroke, try your best to paint in a direction that the shape your painting dictates.
In the red example (see tech. 2) this shape was colored in as if it were a child's coloring book. I did my best to try and stay in the lines and create a nice clean edged shape. By doing this, I had to use brushstrokes that lacked reason, because my only intention in this example was to fill the shape with color. This lack of reasoning may cause a bothersome feeling about your image which if carried through an entire painting can cause a viewer to feel an uncomfortable disturbance they can't put their finger on. A viewer will feel like there's something artificial or incorrect in your image (as evidenced by the messy shapes created by the darker red areas). This negativity then can potentially emphasize or reinforce any other negative feelings a viewer may have, but may not actually be aware of and cause your image to be unexplainably disliked.
I suggest approaching your physical brushstroke as I did in the blue example (see tech. 2) Notice the stroke follows the flow of the shape it's defining. This flow will give off the impression that there is a nice fluidity to your paint application. It will cause the viewer to feel comfortable with what they are viewing. Your paint application will appear as if there were an intention to your whole approach, and it may even cause additional positive reactions for a viewers experience. As in real paint, this technique can be equated to the "paint through an object" theory. If you have ever had any experience with traditional paint (see what I mean) you will more than likely have heard this expression at one time or another. Paint through literally means that. Paint through a shape, do not concern yourself with maintaining edges. (see tech. 3) (this should only concern you when you reach the final stages of detailing) It will also emphasize the theory of leading the eye as explained in the blocking stage. Paint direction can cause a viewer to look where you want them to.