My pediatrician told me the secret to getting our daughter to sleep all night.
At night get your baby sleepy but NOT ASLEEP and put her in her crib with nothing in it. Let your baby fall asleep on her own.
When your baby wakes up in the night she will remember what makes her go back to sleep. Is it nursing to go to sleep? Patting on the back? Rocking? And she will not be able to go back to sleep without it - waking you up crying until you come in and do "that thing". My doctor told me that a lot of nursing mothers think their babies wake up in the night hungry but just nurse for a few minutes and go back to sleep. It isn't hunger, the baby associates nursing with what makes her go back to sleep. If your baby wakes up and know she doesn't need anything to fall asleep, she will fall back asleep on her own.
We've done this and our daughter slept all night starting at 6 weeks old. We may be just lucky but our doctor swears by it!