Critical, or sharp, focus is one of the characteristics of a professional-quality photograph. A poorly focused image is a sure sign of an amateur.
First, check your camera's Diopter adjustment (see your camera's manual). This adjustment helps bring the focus points into focus within the viewfinder. The default adjustment on my Canon 60D was incorrect, and I could only focus properly if I removed my eyeglasses. This was not a problem on my old film camera. After adjusting the Diopter on the 60D, I can now focus through the viewfinder with my glasses on.
Auto-focus (AF) vs. Manual Focus (MF)
Auto-focus lenses typically have a very short range of motion in their focus adjustment. This means that once you have your subject in focus, it is very easy to knock the lens out of focus. The older lenses with the split-prism focus have a longer range of motion. A Katzeye screen can provide a split-prism type focus for many DSLRs (http://www.katzeyeoptics.com). Without such a screen, it is sometimes better to use auto-focus with AF lenses,taking care to select the proper focal point in the viewfinder before taking the shot. For information on selecting the focal point, see your manual. This is incredibly important with a narrow field-of-view shots, but also important with higher f-stops.
Some people have found that adding a 4x or 7x loupe behind the viewfinder helps them to focus better. Some manufacturers, such as Pentax, sell a magnifying eyepiece. There are also specialty focusing devices available for use in capturing astronomical images. Using Live View appeals to some people, especially for studio work, but be aware that the LCD screen only shows a low-resolution preview that uses a JPEG image.
Whether using AF of MF, take your time and get the focus exactly right on your subject. This will help set your photos apart from those taken with lack of attention to critical focus.
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