Heat up the Love
Another theory claims that girl-producing sperm prefer, or can at least tolerate, warmer temperatures. If you buy into this concept, you should encourage your partner to take a hot bath before intercourse if you want a baby girl.
Whether this works or not, don't overdo it. General advice for increased fertility is to keep those guys on the cooler side. So frequent hot-tubbing or overly brief briefs may have a long term deleterious effect. Overall sperm count changes over about two months.