Getting the sex you want?! Is it possible?

GF
Hi, I always hear about myths or treatments ect, on having a boy or girl and how you can? Is it true?:) we have twin girls right now. And when we try again it will be our last, but wanting to try for a boy. Is there ways to get the Gender you want, like having sex on certain days. I know that boy sperm is faster than girl sperm, but doesn't live as long as girl sperm. Did any of you actually try for a certain gender? And actually worked?, if so how? Just wondering:)? Iam happy with either or, boy or girl:). Any advice or stories could help:) thanks a lot!