I really only wanted a third child if it could be a girl. I had 2 boys & had post natal depression after my 2nd. I found the boys hard work together although now i know they're just normal boys.
My friend had 2 boys then a girl & her friend had helped her. Everyone they advised, apart from one woman who had sex too close to ov'ing, had a girl. My friend sent me a testing kit, not sure what it's called but basically to test when it's safe & unsafe to have sex without contraception. It gave me more of an idea when I was ov'ing too. Then my friend just said to have a lot of sex & to stop 3-4 days before ov'ing. I never thought I'd get pregnant that way as previously had sex on ov'ing thinking it was the only time to fall. It makes sense to me why I had 2 boys. You probably know the theory about the male sperm swimming faster to get to the egg 1st. So if you're ov'ing the male one gets there 1st & you have a boy.
Having lots of sex is supposed to get rid of most of the male sperm, then stopping sex 3 days before the male sperm will die because they swim faster so they get there before the egg has been released. The female sperm however can live for about 72 hrs & are slower swimmers, so by the time they get there the egg has been released & you could get a girl.
On the way to my 20wk scan I kept telling myself it would be a boy. It was, & is a girl & we all love her 
That's the only thing we did thou. I know there's lots of other things like changing your diet, getting your man to wear boxers etc.
One other thing which actually is very important to me. I am a Christian & prayed about my deep desire to have a girl. Previously my periods had been irregular but after praying & having someone pray for me, I noticed that I was spot on cd34 every mth for 5 mths. God may have just given me my little girl. But to me it's like He knew I needed to have that bit of input too, I couldn't take the risk so followed my friends advice. To me it makes sense.
Good luck, but like everyone will say, you need to be happy whatever, and I'm sure you will be xxx