Blurry images are caused by many things. The most common cause is probably photographer movement. This is exaggerated by high-resolution sensors such as the one on the just reviewed Nikon D7100, long lenses and dim apertures on typical kit lenses or high-magnification ones.
For better stability while shooting hand-held, turn stabilization on and use this hand-held shooting technique:
- Assume a stable posture, usually not leaning in any direction.
- Support the camera’s weight with the left hand.
- Grip firmly with the right but let the index-finder loose.
- Press the shutter-release halfway and wait for a focus-lock (When using AF)
- Breath in
- Gently press the shutter-release fully.
- Wait for the photo to be taken (Most people skip this step)
- Resume breathing.
A tripod is always best for stability but they can be quite cumbersome, impractical or outright forbidden is some places. Depending on the weight of your camera, a Gorillapod can possibly do. It is a small, cheap and relatively light (How often does that happen?) flexible tripod.