This is quite an issue in my house right now because my youngest will be 4 months old in 2 days. And because we're dealing with it right now, I figured I'd share with everybody.
There's been a lot of controversy on the subject of getting babies to sleep. Pull out a copy of Babywise and you're likely to get a minor war in your living room. You'd think we were discussing relations with China or the deity of Christ for crying out loud. I've never read Babywise (if you haven't either, or you don't know what the controversy is about, just go to Amazon and read the reviews). The premise is that you put your baby on a schedule and stick to it. Some say it's a miracle, some say it causes dehydration, weight loss and failure to thrive.
My theory is probably similar, but like I said I've never read the book. Here's what we do:
First, wait until your baby is old enough to learn to put himself to sleep. For me this is usually around 3-4 months. By this time I know the tired cry, the hungry cry, the hurt cry, etc. He's also pretty much sleeping through the night, usually waking only once. And I've gotten to know about how much sleep and food he needs each day.
Now the very simple but very difficult part. The next time baby is getting sleepy, go put him in his crib and WALK AWAY. Do not rock, cuddle or nurse him. Turn off the monitor. (This is so you can't hear the crying. It'll only make you feel bad. Trust me.) Yes, baby will probably scream and cry, especially if he's used to being held until he falls asleep.
After 10 minutes or so, go check on him. He'll probably be sound asleep, having cried himself out. If not, go rub his back, talk to him and help him calm down but DO NOT pick him up. If you do you'll teach him that screaming gets him back in mommy's arms, which is what he really wants. Once he's calm, leave the room again and give it another 10 minutes.
You're teaching him to fall asleep all by himself. Some people feel this is cruel and heartless. I disagree. I'm giving my baby a valuable baby skill. It's his very first tiny step toward independence. And it means that someday, when he's tired and no one is available to rock him to sleep, he won't suffer from it. He'll just go to sleep, all by himself.
I don't do a real strict schedule until they're a little older. At about 6 months I start to determine what time naps are and when we go to bed for the night. Now that baby knows how to put himself to sleep, I just put him in his crib at naptime. He might go to sleep right away, he might not. But he's in his crib for rest time at the same time every day. One thing I never do, however, is wake up a baby. He'll wake up when he's had enough sleep.
Once we start this schedule, if he wakes up in the night we let him cry. This is hard, but by this time I know when he's crying due to hunger and when he just wants to be cuddled. We don't cuddle in the middle of the night. Again, this seems kind of mean, but really I'm doing what's best for him. He needs a good night's sleep even more than I do. Sleep is vital for baby's little bodies to grow. After a few nights of crying, he's learned that if he wakes up in the night, he goes back to sleep.
Self-reliance. It's a beautiful thing. And it's the very first character training you'll do for your child. Remember that your time raising your child is essentially spent very slowly moving responsibility for their life off your shoulders and onto theirs in tiny increments. Sometimes it's hard to do, but it's best for them.
As for those who say my baby will feel abandoned and alone by this, I understand your sentiment. Sometimes I feel that way, too, when I'm listening to him cry. But 5 minutes later he's drifted off to sleep and when he wakes up bright eyed and happy I know he's okay. I've trained 4 kids this way and they're all happy, well-adjusted kids now who know they are loved.
A few tips:
Make sure when you put baby down that he has a dry diaper and a full belly. Nothing disrupts a nap like a diaper change or feeding.
Keep track of how much sleep baby is getting. If he's crying a lot when you try to put him down, he might not be tired. Or he could be over-tired. I have found my kids have a really hard time falling asleep when they're over-tired and that's when I spend extra time rocking or cuddling.
Be flexible. If you find you just can't listen to him cry, then don't. Just remember that at some point it's got to happen, or you'll be getting up in the night with your 5 year old. Believe me, kids don't just 'figure out' most things, they've got to be taught.