Composition tells us that sharp element in a picture draw the attention over blurry element. By controlling sharpness, you control your viewer attention. There is multiple ways to control sharpness, one of the preferred ways is to use a large aperture. A large aperture leads to a shallow depth of field. Using a DOF calculator you can check in advance which part of the picture will be sharp. You can also pre-visualize sharpness using the preview option on your DSLR camera.
Another option is to used a specialized lens that allow you to control sharpness. A prime example of this is Tilt-Shift lenses. By tilting the lens, you change the plane of focus and can then control which part of a picture is sharp. You can create sharpness zone greater than usual or smaller than usual.
Using a Lensbaby to help composition.
Another option is a lensbaby. This is what I used in this picture. The lensbaby optics are purposefully of lesser quality. Sharpness is not the prime goals of Lensbaby. In this example I used a double optic lens and I tilted the lens on the upper right. This translate into a sharp zone that is in the upper right of the frame.
With the lensbaby you can use aperture rings from f/2.0 to f/22. As the aperture size reduces, the sharp zone increase until diffraction occur at f/11 or smaller.
In this example I chose a large aperture ring of f/2.8. Doing so, the sharp zone reduces. I was on my way to Claude Monet garden in Giverny and these wisteria were ideally suited to create a picture in line with Monet arts.
The blurred background on the left side is perfectly in line with the main subject, these wisteria flowers. Lensbaby is relatively inexpensive. When you get the Lensbaby composer, you can then add various optics to create different look with your pictures depending on the subject.
I urge you to explore the out of focus zone, whatever the techniques are available to you. Like light illuminate and shadows define, using the out of focus areas will help direct the eyes of the viewer on the sharp subjects.