Babies
The first step towards making your baby more confident is by increasing his sense of security. Hold him close to you often. Remember, babies get frightened and anxious very easily. Try and quell his fears by being there for him. You don't have to pick him up whenever he cries. At times, try and soothe him for some time by just patting him without picking him up. If this doesn't calm him, you can hold him. Keep doing this until he starts feeling calm as soon as you pat him.
Toddlers
Your child's brain is developing rapidly, and he understands much more than you realise. He follows your moods, and knows when you are angry, and when you are being loving. He also realises when you shrug him off because you have more important matters to take care of, so be tactful when dealing with him.
Tell your child that you love him as often as possible. Hug and kiss him at every opportunity. Children who are hugged often by parents, tend to be more physically demonstrative when growing up. On the other hand, a child that hasn't had much physical contact with parents is more inhibited and will tend to feel uncomfortable with even normal forms of physical contact, like a friend putting his arm around his shoulders.
Encourage your toddler to do things by himself. Even tying his own shoelaces can give him a sense of pride and achievement. True, it is faster if you do it for him, but do let him try, and praise him even if he doesn't get it right. Also remember that toddlers often get frustrated because there are so many things they want to do, but are unable to. Try and step in before this frustration sets in, and set it right for them. This will not be possible all the time, but do step in tactfully whenever possible.
Listen to your child when he is talking, even if it is all gibberish. Your child understands when he is being paid attention to, and when he is being ignored. You don't want your child to feel that being ignored is something he should get used to or take for granted.
Encourage your toddler to mix with other children his own age. In this way you will ensure that your child will get enough attention for some time from his friends. He will have someone to 'talk' to, and someone who will listen to him.
Pre-schoolers
Encourage your child to do things on his own, and praise him for trying, even if he doesn't get it right. Children often want to try to do a lot of things around the house, but parents don't let them get involved, and prefer to let children concentrate on playing with their toys. This is not always the best approach. You don't want to give your child the impression that he is of no use. Always accept your child's help whenever he volunteers. If you are cleaning up the kitchen and your child wants to get involved, give him a few spoons to wipe. Let him fold some napkins. Then, praise him for his efforts.