C.R.
I've heard the longer btw kids the greater the chance of a boy, so you're right on track by starting now. Unfortunately, as you age your chances of having a boy also increase.
This site has a bunch of methods to try: http://pregnancy.lovetoknow.com/wiki/How_to_Conceive_a_Girl. If you Google "how to conceive a girl" there are tons of sites out there with advice.
If it's really important to you, you can have a girl 100% guaranteed if you choose to adopt. Not only would you save a little girl, but you would have a particularly close and special relationship for having given each other what you'd wished for. You'd also have that great mother-daughter bond when she has your grandchildren. If you aren't successful conceiving a girl this last time, I hope you don't spend the rest of your life with regrets -- you can have a girl if you really want through adoption. Best wishes and good luck.