Best results when changing RGB to CMYK

I agree with Marco U.

I might have said: only use Perceptual if an extremely important color in a photo changes terribly using Relative Colorimetric -- otherwise use that. The reason: The former method can make the whole picture shift (usually yellower and weaker) but the latter can make several distinctly different colors (like 3-5 different reds for example) and make them the same. Usually, one or two really strong colors can get shifted somewhat, but won't be noticeable in an otherwise great looking image.

The secret to converting for good print color is to choose the source and target correctly.

If you are serious about color, buy the book "Real World Color Management" by Bruce Fraser, Chris Murphy and Fred Bunting.

http://www.amazon.com/Real-World-Color-Management-2nd/dp/0321267222/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1198178251&sr=1-1

Sadly, Bruce Fraser passed away, so the book may not go to a third edition.But you can still get the second edition and the first 5 or 6 chapters are an education in themselves.

To convert an RGB file to a target CMYK space, open the image file in Photoshop, then go to Edit > Convert to Profile.

Once there, select your target profile (a copy of which should be in the [Your Computer] > Library > ColorSync > Profiles folder, or else it will not appear in the pulldown menu).

Then select a rendering intent appropriate for your image (to simplify, Perceptual if the relevant image colors are out of the target's gamut, otherwise Relative Colorimetric). If you select Relative Colorimetric, it's also a good idea to check "Use Black Point Compensation".

Do NOT use the Image > Mode > CMYK command to convert your image, because that does not offer the degree of control that you have with Convert to Profile, and limits you instead to the target space and conversion controls set in the application's Color Settings, which may not be appropriate for your purposes.

There's more to all of this, of course, but that would be a good start.

Photoshop - Illustrator - InDesign and respect your colour profiles (sRGB <> AdobeRGB etc).

Lucien.