There are some genetic problems that only exist on boys. If both you are your partner are carriers of these genetic problems, it may decrease your chances of having a boy. Genetic testing is the only way to determine these problems and most of them are very rare. Another problem could be with your partners sperm. Male sperm are generally weaker than female sperm therefore making the probability of having a girl higher. He may want to have his sperm tested to see if his count and viability are normal. Good luck!
I don't think there is a test like that. Many women have taken baby aspirin if they've had multiple miscarriages or have been prescribed progesterone supplements. Talk to your doctor and maybe you can come up with something.