Yes, herc's right, you're supposed to do it every time they wake up. But, frankly, for me it depends upon what kind of cry it is. Sometimes my dd will wake with a shrill, startled cry like she's had a bad dream, and I find that if I go in quickly then and shush her, she never really wakes up.... CC is not an easy task, but she's been waking once a night now, rather than four or five.
When I go in to her, I try not to speak to her too much, but rub her back gently and say, "Go to sleep, xxxx. I love you." The idea is that you leave them for increasing intervals (i.e., 5 minutes, then 10, then 15, etc.).
CC is certainly not for everyone, but I've found that it helps us a great deal.