Written for BabyCentre UK
Penney Hames answers:

For the first six weeks -- and sometimes 12 -- most babies sleep erratically. You are lucky if you see a pattern emerging before this time. But after three to four months you can help your baby into a regular pattern of sleep so long as you act consistently and positively.

It can be difficult to know what to do for the best, especially when there is so much advice available. But you are the expert on your baby, and you know what will work best for your family. So, decide what you want, and then go for it. Babies are remarkably adaptable -- you can teach your baby to sleep without you or with you, in your bed or his cot. The choice is yours. But you are much more likely to stick with a plan that feels right for you than one you happen to find in a book or magazine.

Every time your baby wakes during the night, you will need to repeat whatever you do to help him fall asleep for the first time each evening. If you feed him to sleep, he will need you to do the same every time he wakes up; if you leave him alone to sleep, he will expect that.

There are only two essentials for any plan:

1. Once you begin a course of action, see it through. If you decide that you are not going to rock your baby to sleep, but instead place him in his cot sleepy but awake and return to whisper reassurances to him every five or ten minutes, don't give in after 45 minutes -- you will merely have taught your baby that it is worth his while to persist for as long as possible.

2. Give your plan time to work -- at least a week or two. Each new plan that you try means that you are asking your baby to learn a whole new set of sleep habits. Old habits take time to disappear, and new ones time to become established. Don't confuse your baby by chopping and changing. Stick with your plan for at least one or two weeks.

Your baby will be soothed into sleep by a predictable and relaxing bedtime routine, and you will be more ready to say good-night once you have spent some time being close with him. He will also be more likely to sleep if you try to avoid letting him fall asleep in the late afternoon.

If your baby still does not sleep in spite of your best efforts, you may find it useful to talk to your health visitor.

Source: http://www.babycenter.ca/baby/sleep/wontsleepexpert/