Many babies and young children develop "sleep crutches" or "negative sleep associations."  They use this crutch or association to get to sleep -- and then they need it again when they wake up at night, meaning that when they experience those normal partial arousals we all experience during the night, they can't get themselves back to sleep without you coming to supply the crutch.  The behavior by itself isn't negative -- we should nurse, rock and snuggle our children.  What's negative is how the child associates that behavior with falling asleep, and how they can't get to sleep or back to sleep without it.  Your job is to help change that association in their little minds.  Here are some helpful tips: 

Bedtime feeding:

Nursing or bottle feeding your 6-month-old or older baby to sleep is only a problem if it is the only way you child can fall asleep or get back to sleep at night.  If your child is nursing himself to sleep at night, or in the middle of the night, or just briefly suckling at night to get to sleep -- or if you have begun to feel like a human pacifier -- it's time to address this.  This does not mean you stop breast-feeding or eliminate any necessary bottle feedings.  But you should modify your routines so that he isn't falling asleep on your breast all the time or with a bottle in his mouth.  Once your child begins to break that mental link between eating and sleeping, he will more naturally and easily wean himself from middle of the night feedings, and sleep longer. 

Here's one way to get started.  Let's say your baby's pattern is to drink for 20 minutes, then suckle on and off while dozing for 20 minutes.  You should try to cut the time to nurse in half -- to a total of 20 minutes.  Unlatch the baby when he has stopped drinking and has begun to fall asleep.  Burp, swaddle, kiss, sing, or briefly rock her, then put your baby in bed while drowsy but still aware he is being put down.  Sit next to the crib and offer verbal and physical reassurance.  When he wakes during the night for a feeding, quickly attend to him.  If he falls asleep while nursing, unlatch him and return him to bed.  Focus on putting your child down drowsy-but-wakeful at bedtime, and for at least one nap for 7-10 days. 
If you and your pediatrician have decided that your child is big enough and healthy enough to go all night without feeding, you should provide comfort but not food as he adapts to the new routine.  When he awakens during the night, go and sit next to the crib but don't feed him.  Do this until he falls asleep at each awakening.  Some parents find that sleeping on an air mattress or other temporary bed on the floor of their child's room for the first few nights make them and their child feel more comfortable.  But don't do this for more than a few days.  Every 3 days, move your seat a little further from the crib, toward the hallway.  You can make lots of soothing sounds so he knows you are there. 
A lot of families find that if the Mom is nursing, it's a good idea for the dad to go sit by the crib in the middle of the night, because the baby won't expect to be nursed by Daddy.  Follow the same logic with bottle feeding -- if Dad is the one who gives the bottle in the middle of the night, when you are cutting out that bottle, have Mom be on crib-side duty.

Walking or rocking to sleep:

Walking or rocking a baby to sleep can also be a crutch, or a negative sleep association.  A baby can't get to sleep, or back to sleep, without being walked or rocked.  You don't have to abruptly stop it -- that might be hard on you and your baby.  Instead, gradually cut back.  If you walk or rock your child for 30 minutes until he is completely asleep, try 20 minutes for 2-3 days and then 15 minutes for 2-3 days, and then 10 minutes, and so on.  Your goal should be to put your child down when he is drowsy but still a little bit awake, so he learns to do that last part of falling asleep on his own.  Work slowly toward putting him down in his crib or bed in a drowsy-but-wakeful state at bedtime.  Then work on all other night awakenings and the naps.  Follow the same guidelines mentioned above for crib or bedside reassurance. 

If you decide to pick the baby up when he is crying in the crib, pick him up only long enough for the baby to calm, and then put him back down and continue to be reassuring.  Many babies like to be swaddled tightly because it feels like being held, or they may like to have the bumper of their crib touch the top of their heads.  As you work on this transition, you might want to rock the baby's body gently with your hand after you put him down -- but don't do it for too long or else it may develop into another sleep crutch.  Whatever walking and rocking you do should be in the bedroom, not all over the house.  You want the baby to create positive associations with his bedroom and sleep.