You cannot do anything in your sexual habits to change the sex of your baby. All men who produce viable sperm produce both male and female sperm, and both have an equal chance of fertilizing the woman's egg. Through in-vitro, however, a doctor could take a female zygote and implant it into your womb.
There is a technique called "sperm spinning" which can influence the gender of a baby before conception. Most methods of trying to influence baby gender need to happen before conception. There are other methods that claim good results (70%+) as well, but they are often trade secrets.
You cannot choose which sex your child is going to be. Although, by consulting a doctor, they can do a special 'procedure' which can make you child which sex you want it to be. I wouldn't suggest it as having a baby is one of the most enjoyable things of life, and no matter which sex of a baby you get, you will learn to love and cherish them more than anything else in the world, and you should be happy to get either a boy or a girl.
There are no trade secrets in determining sex. If you want to make your baby a girl after the baby is born, then treat the baby like a girl. You may not have the desired results physically, but that can be corrected later on in life - I am assuming you come from an affluent country where you want girls, and therefore such operations are available. Until the baby is ready for the operation, you will have a girl in spirit. You'd be a terribly awesome parent to try and attempt this.
If you are completely desperate to have a girl, adopt one. If you want to have the child yourself, be prepared to have a boy or a girl. You can't choose the sex of your baby. I heard about some thing they have going in Russia but that goes wrong a lot and can result in a hermaphrodite.