As you probably already know there is no 100% way to guarantee you will get the desired sex you want. However, I have found a few tips online you may find interesting that will put the cards in your favor, and increase your chances of having a boy.
http://pregnancy.lovetoknow.com/wiki/How_to_Conceive_a_Boy
I can't remember if this is the site that told me this, but let me add to this answer because we have a girl too and wouldn't mind if we have another girl, but a boy would complete our family better.
I found some information online about two years ago and it advised that the woman's vaginal tract be more acidic for a girl, and more alkaline for a boy. I found this out after my daughter was conceived and bought some PH test strips online, which test your urine or saliva (Alkalive PH testing Strips). Not knowing I was already preggers I tested and I was clearly more acidic.
Your PH balance probably changes slightly when you are preggers, but I thought that was odd when we found out we were having a girl 4 months later!
This is not the site I found the info. on but this is some information on how to make you more alkaline and imorove your chances of having a boy:
http://www.ehow.com/how_5046058_make-yourself-alkaline.html
http://www.livestrong.com/article/16383-make-body-alkaline/
The information I found explained it like this: The male (Y) chromosomes on the sperm are faster but weaker and once they enter the woman's vaginal walls they will die first as they can't survive as well in an acidic environment.
The Female (X) chromosomes, the "female type" sperm are slower, but stronger.
If you want a boy you can modify your diet to make your tract more alkaline, which is actually more healthy. I found an alkaline diet online and if I find it, I will add the link to this message.
The other thing is if you orgasm, it makes your tract more alkaline so that will improvce your chances of having a boy.
Since the Y Chromosomes are faster, the deeper the penetration, the better. It is advised that the best is the position in which you are on all 4's and he is behind you. I don't want to call you from it's slang name, but you know what I'm talking about!
I have never tried these personally, but it's worth a try! I hope this helps and I will send you more info. if I find it .
Good luck conceiving your boy!