Such a great questions, one that I struggled with in the past as well! In general, the two leading causes of blurry images are:
- shutter speed is too slow
- depth of field is too shallow
With shutter speed, I would recommend not letting the shutter speed drop below 1/150th of a second. By keeping the shutter speed above this number, you should be able to remove any camera shake that may be costing you sharpness.
To learn more about shutter speed, check out my article, “The Ultimate Guide to Learning Shutter Speed.”
The other common issue that causes blurry images is that the depth of field is too shallow. A good rule of thumb is to never shoot at an f-stop smaller than the number of people in the image. For example, if you are shooting a group of 5 people, shoot at f5.6 or above. By following this tip, you will make sure that everyone in the group is sharp in the image.
To learn more about Aperture and F-stop, take a look at my article, “A Beginners Guide to Understanding Aperture.”
Another great article to read on the subject is, “Ten Tips for Getting Sharp Images Every Time.”