The biggest is color management by far. I use the term "photolab" instead of "online printing company."

• Ensure your files have the correct color space attached. Many pro labs support a variety of color spaces, so you should use the one that they recommend.

• Have your monitor color calibrated using a hardware colorimeter. Eye balling it, will not give you the best results. When color calibrating it, you need to make sure that the ambient light levels are consistent with how you typically view and edit your photos.

• Once your monitor is calibrated, you need to obtain the ICC color profiles for the printers from the photolab to which you will send your photos too. With the correct ICC profile, you can "proof" your photos using software which will show you how you can expect your photos to look.

• Above all else, contact the photo lab and find out what steps they recommend for getting the most accurate colors from their prints.

• If by chance, the photolab does not supply ICC profiles, and you have your heart set on still using this lab, you can send them a color calibration target to print. You can then send this image to a specialist company who will build you an ICC profile, and then proof the images. This of course is a royal pain, so you might as well just find a different lab to do business with.