Backgrounds in watercolor, using glazing techniques can be remarkably simple, and easy, but yet also challenge the eye and make the viewer focus on your main subject. Here's a couple of tips to help you make your backgrounds stand out better, especially when painting vignettes.
First, always use good professional pigments and good quality paper. There's nothing like painting something really great using cheap equipment that isn't archival in nature. For pigments, I like M. Graham, Holbein and Sakura. I have a tendency to stay away from Windsor-Newton, despite it's popularity, I find especially in M. Graham and Holbein that the pigments are ground much finer and the binder is of a higher quality. Avoid student paints like the plague. For paper, I like 300 lb Arches, which I usually buy in the full sheet and cut down myself, depending upon the size of work you're after. Often times I work on watercolor blocks, which are very convenient for plen aire work, and pack up well. Do be warned, however, if you work strong wet-into-wet on a block, the last few sheets will have absorbed so much moisture, the sizing will be destroyed. Avoid cheap paper without sizing on both sides. Fabriano is also another excellent brand, and if you want to try something really different, Yupo is a great alternative painting source.
Good brushes. Can't say enough about this. I prefer the inch angle shader for most of my glazing work, then I work up to or larger if necessary. I don't always buy natural bristols, but often times good synthetic ones will make this task much easier. Make sure, when you buy brushes in the store that the flat of the brush is even and people having been mashing it about. Always take good care of your brushes, let them dry out, after rinsing with clean water and using your fingers to shape the brush to get a longer life expectancy. I store mine bristle side up in a jar.
So now the work. Let's say you've been working on a nice floral piece, a red rose. So you've got your rose, and all this white paper staring up at you.
Now ideally, you'd use a contrasting color to make the rose, 'pop' up against the paper, but roses generally contain stamens and green leaves, so using green might not be the first choice. However, if we were to select another color, say, a light blue, we get the contrast of warm rose vs cold background, and they will automatically compliment one another.
So make yourself out a puddle of blue, but go heavy on the water. The trick here is we're using a glazing technique. Grab your brush, and go slowly around the edge of the rose. You've created what's called a, 'hard' edge, meaning the rose is now defined.
Now go around the border and do the same thing, drawing the pigment to the border around the rose. You'll find if you work heavy wet-into-wet you'll get a lot of what's called blooming. A perfectly natural occurrence in watercolor. If you don't like it, while still wet, just mix it about a bit, and it'll go away.
So now you've got the beginnings of your background. You can, at this point, probably leave it be. I like to go with 5-10 glazed layers which create a darker aura around the rose, and around the edge of the picture, but still leaves a slight, 'glow' that's roughly in the shape of your piece. The human eye will take that, and imagine all sorts of shapes in it, because you were working wet-into-wet and that's the nature of watercolor.
You can call it done, or maybe add a new color to your background. What happens there is you start to trap the eye as it's trying to decipher what are all these colors. I don't go too far with this, as you don't want to ruin the piece and make mud. Mud making bad, good, thin translucent background is what you're after with this technique. Hope it helps!