One of the main challenges of a photographer is to find the precise moment to release the shutter and capture the "decisive moment" that best describes the essence of a scene or simply gives a good design.
When photographing landscape or still life scenes, you generally have sufficient time to find some means of emphasis in your shot with the use of lines , tones or subject positioning within the frame.
But if things are active in front of the camera your choice of moment to shoot and capture the peak of the action can be quite tricky. Your best choice might be to find the most promising viewpoint for capturing the visual climax to the action and just wait for the right moment. You can first frame up a background shape or foreground lead-in for example, and then wait patiently for someone to enter the frame.
In other cases, your scene might be full of moving objects surrounding some relatively static element. Having framed up the scene, the choice of moment to shoot is dependent on the various subjects in the scene. You will need to wait for the right moment when the changing forms and patterns achieve balance and for an instance create a picture.
"Our eye must constantly measure, evaluate. We alter our perspective by a slight bending of the knees; we convey the chance meeting of lines by a simple shifting of our heads a thousandth of an inch... . We compose almost at the same time we press the shutter, and in placing the camera closer or farther from the subject, we shape the details - taming or being tamed by them."
Capturing fleeting comparisons which support and draw attention to your main subject can also make for very interesting and effective images. As in the case of the below image showing a row of ducks:
Another way of achieving a similar effect is by showing two different "compartments" within the picture. For example, comparing people framed in adjacent windows of a crowded bus.
A useful and effective way of bringing two quite separate components together into your image is with the use of a reflective surface or a mirror as seen in these images :
To summarise, in order to capture decisive moments you need to be focused on or receptive to what is happening around you rather than the camera equipment you are holding. You must spend time to familiarise yourself with your camera equipment so that you are able to operate it intuitively and without looking at the controls be ready to make your choice of moment to shoot.