First, some of the suggestions we have been given over the years...
For a boy — have rear-entry sex standing up, when you're both stressed out, at night, on an odd day of the month, when there's a quarter moon, with mom's head pointing north, and make sure dad's testicles are warm. Lay down afterwards... and make sure it's to the left of dad!
For a girl — have missionary sex with mom on top, when you're both relaxed, in the afternoon, on an even day of the month, when there's a full moon, and make sure dad's testicles are cool.
For a boy — eat red meat, salty stuff, calcium and magnesium-rich food. (Dads, drink soda — especially cola.)
For a girl — eat chocolate, sweets, potassium, and (both mom and dad) lots of fish and veggies.
For a boy — dad should suggest sex, climax first and be the dominant partner in the relationship.
For a girl — mom should suggest sex, climax first and be the dominant partner in the relationship.
Some of the theories our users have heard...
"The rule is this: On top = a girl, missionary = a boy." -elevendiamonds
"Doggy style makes boys." -Bonzer
"I've heard that if your DH is 'pretty' then you will have a girl first, and if he is handsome (more rugged or not as pretty) then you will have a boy first. We are having a boy..." -Bridey1
"I heard it's the 'type' of sex you had when conceiving. Wild = boy, tame = girl." -littlecindy
"I have heard about for a boy, doing it day of or after ovulation. My FIL always tells us that my DH was his 'best swimmer.' We are going to try it actually! We have a DD, so now we are on for DS." -Mandy8
"Someone asked me the other day if I had a 'feeling' one way or the other on the gender. I said I was leaning towards boy. She asked, 'Oh, did you eat a lot of oranges before you conceived?'" -Dobie_Mama
"We went for a sports theme: My husband wore sports equipment (shoulder pads and a football helmet) and I wore my old cheerleading uniform... Not! Role playing is one thing, believing that it will determine the sex of your child is another." -Doodlebugz
"Our mom group leader has a theory — it was true for her five kids, our DD and a friend. She said that if you have sex before you ovulate it will be a girl, and if you have sex when or after you ovulate it will be a boy. She said that girl sperm live longer than boy sperm but boy sperm swim faster than the girl sperm." -mommycj