Okay, found it. This is from a book by Niels H. Lauersen, m.d, ph.d and in his chapter on
sex and your baby's sex, it says (kind of long so bear with me):
"In addition to affecting fertilization, some research suggests that how you make love can also influence the sex of the child you create. This is said to be due to differences in the genes carried by sperm. Sperm is divided into two types, those that carry the genes for a female child and those that carry genes for a male child.
MALE SPERM ARE SAID TO BE
*fast moving
*good swimmers
*small
*easily affected by acidic vaginal secretions
*short lived
FEMALE SPERM ARE SAID TO BE
*slow moving
*large and hardy
*able to withstand acidic environments
*able to survive longer in the female body
According to the sex selction theory, certain sexual positions, along with the depth of penetration and when in relation to ovulation you make love, can be used to encourage fertilization with either male or female sperm. While there is no guarantee that sex selection will work, many of my patients have used the following techniques to plan their families with incredible (80% +) accuracy.
If you want a baby girl:
*shallow pentration in the missionary position will allow your partner to deposit sperm at the mouth of your cervix. This is said to favor the slower-moving female sperm.
*avoid orgasm. this will help keep your vaginal environment highly acidic which kill of male sperm before they reach your fallopian tube. to increase the acidity of your vagina even more use a vinegar and water douche directly preceding intercourse... (that's just some of it, I got sick of typing

)
If you want a baby boy
Use deep penetration, and the rear-entry position, which will allow your partner to deposit sperm above the neck of your cervix, for fast entry into your uterus. Here the environment is more alkaline making it easier for male sperm to survive.
*try to achieve orgasm at the same time that your partner is ejaculating. This will also help create a more alkaline vaginal environment.
*your partner should avoid masturbation for four to five days, and you should both avoid intercourse until your day of ovulation. this will help ensure the highest possible sperm count and, toghether with the other factors, give the advantage to the fast-swimming male sperm.
I left some things out but you get the main idea and a few pointers with this.