Some things we know make mosquitoes more attracted to you:
Exercising, higher metabolism, higher body temperature, more body odor, being pregnant, type O blood, infrequent bathing, lactic acid, ammonia, acetone.
There are a number of folk remedies people believe protect them from mosquito bites like drinking alcohol, eating garlic, or taking vitamin B. These do not appear to provide any benefit in lab studies and in fact drinking alcohol is associated with increased mosquito activity because it causes blood vessels near the surface of the skin to dilate.
And apparently some of your attractiveness to mosquitos is simply genetic. This may be mediated through your immune system, which is what a lot of the genes identified were associated with.