Not sure about a special diet, but I have read that if you want higher chances in conceiving a girl, you should start ttc on cycle day 10 (if you are on a 28 day period, or on the day that is six days before you ovualte) through to cycle day 12.
This is b/c the female sperm can last longer and swim further than the male sperm and since you are further away from ovualting, the chances of the female sperm reaching the egg is higher than those of the male sperm. (the same would go for if you wanted a boy; ttc on cycle days 13 through to the day after you ovualte. They will swim faster to the egg!!)
hope this all makes sense, and good luck!!!

I heard the same thing as jaemierin said. I read it on webmd actually and they made me a calendar on there that I was supposed to go by if I wanted a certain gender. To me, I do not care what the gender of my baby is, but sometimes people do. The gender of the baby is decided by the father's genetics so if your hubby or whoever has a high number of girls in his family then you have a good chance of conceiving a girl.