Blur caused by hand shake is the number one killer of good images.
Blur can be prevented by using a high enough shutter speed to overcome the shake of our own hands. The rule of thumb is to use a shutter speed that is at least as high as the highest focal length of your lens. So if you’re using a 50mm lens (Which creates an image that is just a little longer than normal.), you need to shoot at a 1/50 of a second. A 200 mm telephoto would need a shutter speed of 1/200 (or 1/250 which is more usually found on cameras).
The bad news is if you’re using one of the semi-automatic or creative modes (aperture or shutter priority or program mode) you have to be certain that the camera is set to a high enough shutter speed to overcome blur. In shutter priority mode it’s easy. You set your shutter speed and the camera sets the aperture. If a proper aperture isn’t possible using the shutter speed you’ve set, most cameras will warn you in the viewfinder. In aperture mode it’s easier to make mistakes as the camera is setting the shutter speed for you.
Some cameras have ability to set the ISO (the sensitivity of the sensor) automatically. This is a good thing when it comes to helping you avoid camera shake or blur. It’s not so helpful when you’re attempting to control your exposure. Then it’s best to set the ISO for the lighting situation you’re shooting in. (ISO – 100 to 200 for best quality; 200 to 400 for overcast days; 400 to 800 when the light is dim; 800 to 1600 for indoor or sports photography.)
A tripod or some other form of stabilizing the camera is essential when taking landscapes or shooting a night. A flash will eliminate blur but often creates an unflattering photograph.