In 2D Applications, brushes can be varied a lot, to resmeble more natural brush strokes, and to give special effects. Lot of folk don't know about these features.

  1) PAINT PROGRAMS HAVE MORE THAN ONE BRUSH!

  Nearly all paint programs have a variety of brushes, you can also get many free or to buy on the web.
You do not have to stick with a round brush! ;)

  Custom brushes can also produce special shapes, such as bullet holes, or are good for painting hair etc.

  You will find the "pen" of a graphics tablet a lot better than a mouse, for anything that isn't a straight line. There are many brands of Graphics Tablets. IMHO, the Wacom ones are best, but Trust and Genius also sell low priced versions.
The Wacom Intuos line gives lots of very nice features, like "Pen Tilt", so the angle you hold the brush can affect painting, which means that "air brush" can actually spray paint in angled directional fans with some apps: very handy!

2) PAINT SHOP PRO BRUSH PALETTE

  This shows where you can adjust the brush to provide random, more natural effects. When set to "Normal", they work off the random percentage set to ther right side (goor for a mouse especially), when set to Pressure, a graphic tablet's pressure on the tablet also affects it, both are combined! You don't have to dial in a % number if you don't want to, and pen pressure will provide variance anyway. Matter of taste, but with this ability, even a mouse can provide random, more naturalistic brush strokes.

• COLOR BLEND alters the special way the brush reacts with what's below, same as altering a Layer mode, from Normal, to Multiply, Screen etc. Don't mess iwth this unless you know what yer about ;)
• HUE will cause randomness in the actual colour! Nice when making some randomness for say, thick oil painting or weathering.
• SATURATION, alters the strength of the colour, from very vibrant (off) to randomly greyish (maximum)
• LIGHTNESS, as name suggests, alters brush colours form light to dark, higher the value, greater the effect
• SIZE, definately one you always want to have with randomness when needing naturalistic looks.
• OPACITY, roughly how thick or thin the paint is, very good one ot have randomly set
• THICKNESS, how thick (horizontal width) the brush is.
• ROTATION, rotates the brush, very good one to have randomized so the brush doesn't appear unnatural and "samey".
• DENSITY, how dense the brush hairs are. Gives splotchy, stippled look when turned on, again more random and natural but not always what you may desire.
• STEP, this is a setting in the main Brush palette elsewhere on screen too, but it's important. "Step" is how often the computer checks to make a brush mark. When set to 1, it's liek you keep the brush constantly on the paper. The higher the number, the less often the computer "puts the brush on the paper", eventually giving a look like you daubed the brush on only now and then as you move.

4) A good collection of free Photopshop brushes on the web, is "Obsidian Dawn". Note that Paint Shop Pro can also use many (but not all) Photoshop Brushes.