Composition is one of those art topics that is independent of medium. At least, in the two-dimensional realm, it doesn't much matter whether you are painting in oil, pastel, tempura, watercolor or acrylics, nor does it matter if you are painting still life, landscape, portraiture or abstract - certain rules of composition will always apply

The Golden Mean

The Golden Mean, also called the Golden Section, the Golden Ratio and even the Divine Proportion, is one of those mathmatical concepts that has a surprising application to art. Simply, it is 1.61803398875... This is one of the oldest known invariant proportions, known as Phi (similar to Pi). It means the ratio of two parts of one whole such that the small piece is to the large piece as the large piece is to the whole.

Each length is divided according to the Golden Ratio and arcs drawn through the resulting rectangles; this is the shape of the nautilus.

But what's that got to do with Composition?

When composing a picture, one should always try to avoid a 'bulls-eye' - centering the object of interest in the exact center of the field of view. If the 'center of gravity' of the picture is at the 'golden mean' along one or both axis, the picture will look more natural and balanced than if everything is in a straight line in the center.

I illustrate by making an arrangement out of clipart bottles. Here's a boring arrangement, with the largest bottle bisecting the picture plane in the exact center:

Here's a slightly more pleasing arrangement, with the center of mass of the objects moved both to the right and down in the plane, so that the 'weight' of the picture falls closer to the Golden Mean.

Admittedly, 1.61803398875 isn't the easiest to remember, nor its reciprocal, (1/x is the reciprocal of x), which is, weirdly, .61803398875 - but it's about two-thirds. So for all practical purposes, you can divide a plane up into thirds like so:

Placing the 'center of mass' of your composition at any of the red dots will make your placement seem natural. A horizon shouldn't split the plane exactly in two, but into one-third and two-thirds, nor should a strong vertical such as a tree or tower occupy center-stage in the other dimension.

A little forethought in the laying out of your painting can save you a lot of fixing later.