I'm not saying it will necessarily work, but my much younger sister wholeheartedy recommended to my dd a book by a Dr Ferber - 'Solve your child's sleep,problems' or similar title. Her dd was still a nightmare sleeper at 18 months and she said the method did work inside a week - she wished to heaven she'd found the book sooner.
He explains how babies and young children get in the habit of always going to sleep always a certain way - breast fed, or walked or rocked or pushed, or even lying on the sofa in front of the TV. So when they wake however briefly in the night they have come to expect the same. So they have to be trained to break the habit and self settle.
Trouble is, it does involve leaving them to cry for certain periods, sometimes going in and patting but not picking up, or otherwise interacting or feeding. (Obviously doesn't apply to young babies still needing nighttime feeds).
And this can be so difficult and stressful for parents - it's awful to listen to your baby crying.
My dd has a very poor sleeper of not far off a year old - always breast fed to sleep and in bed with her, waking several times a night - she is permanently so tired, and has a toddler too, but she's finally beginning to try this way, and it's helping so far. I hope to goodness she will stick to it, because I don't know how she'll manage with little sleep once she goes back to work.
I must say that the current 'wisdom' of never leaving them to cry, even when they are evidently badly in need of sleep, does result in some utterly exhausted parents.