I highly recommend ingender.com if you want to try for a specific gender. You will learn how to sway (a term used for ttc a certain gender) and there are plenty of Mommies on there trying to do the same.
I swayed for my daughter...and she is all girl. My DH has all boys in his family and our other 2 are boys plus I am pregnant now with a boy. I would probably be an all boy mom, if it wasn't for ingender
It is a lot more complicated than just timing. It has to do with your ph levels (acidic vag favors girls, alkaline favors boys), whether you orgasm or not (orgasm helps to make a boy, no orgasm for a girl), the moon cycle (can't remember exactly all the info on that), also you need to eliminate salt and potassium, and load up on calcium and magnesium (for a girl). On top of all this is the timing aspect, as well as DH must either ejaculate every day before hand or not at all for 8 days before (to decrease his semen quality, which give the boy sperm less power to swim. My DH also took super hot baths every day for a week before we ttc to kill some of the boy sperm) and also your ionic charge. You have to do all of this for at least 6 weeks before ttc. I know it all sounds a bit crazy, but there is real science behind all of it...please read all about it on ingender.
Both you and DH must be on board for this. It won't work if only you do the diet. My DH and I always joke that our DD was a science experiment! She was worth it though. (We also say she is worth all the strawberry jam and cream cheese in the world, because that's basically all we lived on!)
All the best! Hope you get a sister for your girl!