How to Get Prints that Look like what I see on my Monitor

I am giving CDs with Xmas card images for clients to take to their local printer and print their Xmas cards. But they are getting different results than what I saw on my monitor -- often much darker. Is there something I can do to be sure that what the lab prints is the same as the image I saw on my monitor???? I have a PowerMac G5 with a 17" monitor, which I calibrated with a Monaco colorometer....I am using Photoshop CS2.