Dot Gain & Gray Balance 
Dot gain and gray balance are the cause and the solution to the vast majority of color problems on press. Together they are very powerful tools for acheiving accurate color reproduction on press. They are also a source of great misunderstanding.
The largest misconception about dot gain and gray balance is that there is one correct standard way for a press to print, and by adjusting a press to match this standard all your color problems will be fixed. The reality is that the correct dot gain and gray balance is determined by the image being printing.
When an image is converted into CMYK, the separations are created with a specific gray balance and dot gain built in. If the press does not match these gray balance and dot gain characteristics, then the image will not print the same color as the original. The problem is that files do not always get converted to CMYK using the same characteristics.
Thankfully most images are converted using the same characteristics without the designer or printer even realizing it. These characteristics are the default color settings in Adobe Photoshop, and they are the most logical goal for the average commercial printer to match. 
Photoshop uses the Specifications for Web Offset Publications (SWOP) as the default CMYK characteristics. By setting your press to match these characteristics, you stand a better chance of matching the color of the original image.
The specification for measuring SWOP compliance on press has changed for 2007. The older method is presented here because it can be acheived with a dot reading densitometer and does not require a spectrophotometer.
SWOP specifies that at densities of K1.60, C1.30, M1.40, Y1.00, a press should print with a dot gain at the 50% of K22%, C20%, M20%, Y18%, and that gray balance at the 50% should be acheived with C50%, M40%, Y40%.
The first step for matching the specification is to adjust dot gain. This calls for printing a complete tone scale at the specified densities, comparing the printed dot gain against SWOP dot gain, and then creating a CMYK plate compensation curve to make your output match the specification.
Once dot gain has been adjusted, color should be accurate on press as long as density and gray blance are maintained.
Density and gray balance are inter-related, and gray balance should take precedence when making ink adjustments on press. A simple technique for monitoring gray balance on press suggested by Dan Remaley, a technical consultant for PIA/GATF, is to use a C50% M40% Y40% swatch in your color bar. This can be read easily with a densitometer set to show all Filter readings. When the measured densities are within .02, you should acheive a nuetral gray balance and be printing accurate color.
