The background is an essential piece of any painting, and should not be considered extra space, or filler for the painting's subject. A strong background makes a stronger painting. For some artists, the most difficult task of completing any painting is keeping the background to the level of the subject, giving it as much care and attention. With practice this will come more naturally to the artist.
Timing
The background should either be painted first, or at the same time as the subjects of your painting. Be careful if you paint the background first--do not complete it before the subject itself has been integrated into the painting. This could give the painting the effect of having disparate parts. The background should have an interaction and relationship with the subjects of the painting. The painting itself should be one harmonious piece, and not less than the sum of its parts.
Perspective
Objects closer to the front will appear larger than objects toward the back. Anyone who has studied perspective should know that the perceived size of an object directly correlates to its distance from the viewer; it is not random. When painting from a live subject or photograph and not imagination, pay close attention to the perceived size relationship of objects in the distance as they appear against objects in the foreground.
For linear objects, such as buildings disappearing down a long straight road, or the dimensions of a room, actual linear perspective should be utilized. In this case, draw the details of the background carefully onto the painting before actually beginning the painting. Linear perspective is so reliant on accuracy that it should not be attempted without use of a ruler and a pencil with eraser.
Creating Illusion of Distance
For paintings that span great distances--like landscapes that may appear to go on for hundreds of feet, or for many miles--there are more tricks to creating the sensation that objects in the background recede further and further into the distance. Note that these tricks generally do no work or apply to objects in the background intended to be close to the subject.
For example, objects will lose clarity. As they get farther away, they will become lighter in color as a result of air quality and atmosphere. This effect can be exaggerated for the purposes of creating a sense of great distance.
Details in objects will become indistinguishable. As a grassy plain recedes into the background, the individual blades of grass meld together to form a patchy, integrated blanket of green on the ground. Leaves on trees are no longer visible individually, instead they are perceived as one great body.
Background Ideas and Treatments
A strong background will contribute to the design and cohesion of a painting. When designing a background, the artist should give consideration to the structure as it guides the eyes of the viewer back to the subject. A background should also be considered as an opportunity to incorporate elements of significance in a painting that will either contribute to narrative, reinforce meaning or build atmosphere.
Different kinds of backgrounds are appropriate for different paintings. For example, a common practice for a portrait is to leave a flat background so that the subject can pop to the center of attention. Also common for portraiture is the placement of stationary objects in the background that may be of personal significance to the subject of the painting.
On the other hand, landscapes are often without a specific subject, or the subject in a landscape is often the background itself. In this case, careful consideration must be given to the placement of all elements in the background, the interaction of sunlight and shadow, and the way that the elements of the background move the eye across the canvas.
