it's hard when it is too late - getting a good night routine sorted is the key thing. i watch for signs during the day, but in the evening, bath and bed time are 7pm - no matter what. it's the only real routine we have - everythign else is so unpredictable. but it's nice to know that from about 7.30 the rest of the night is our own
it took several weeks of crying every night, right at the beginning, sometimes for a long time, sometimes less, and now she settles every night at bedtime with no trouble at all. i would say bed time is the one time leaving them to cry might be ok if everyting is checked - food, heat, nappy etc. certainly i found me coming and going, pick up put down etc just woke her up more
during the day it's different - getting her to settle for naps is a bit hit and miss as i said. if i catch her tiredness too late and she won't settle i get her up again, try and get her tired again (as by then crying and grizzling has woken her up) and then spot the signs early. it can take a few attempts
as oblomov says, the occasional crappy day or night is inevitable - bound to happen