It's well documented (and I've seen it plenty of times with my own eyes) that mosquitoes prefer to bite some people over others. In New Jersey we used to do 'leg counts' to assess mosquito populations, instead of CO ₂ (carbon dioxide) traps. You stand still for one minute and count how many mosquitoes land on your legs. I used to run into a guy from one of the local mosquito abatement districts at one particular location and his leg counts were always much higher than mine at the same spot at the same time of day.  He was a big fellow. 

The amount of carbon dioxide you emit is definitely a factor for 'long range' attraction.  Anything that causes more exertion (exercise, being overweight, pregnancy, higher metabolism, etc.) will cause you to produce more CO _2. Close in, it seems like various chemicals in our breath and our skin play a role in whether we seem tasty to a mosquito. Some of these may be related to blood type. However, there's not just one kind of mosquito, there are hundreds of species and they all may have different preferences ( e.g.,  Culex species prefer birds over people). What attracts one species may repel another. So, one study shows blood type seems to matter to one particular mosquito species and studies with other species seem to show it doesn't matter at all. Final answer: it's complicated.

Regarding reports that beer drinkers get more mosquito bites, mosquitoes are probably attracted because the bubbles in beer contain CO₂ (produced by yeast). There's nothing especially attractive about yeast, it's just a CO₂ source. 

For more information on what attracts mosquitoes to people,  check out this article.

The American Mosquito Control Association states:

What attracts mosquitoes to me?

Why some people seem to be more attractive than others to mosquitoes is the subject of much repellent (and attractant for traps) research being conducted nationwide. Carbon dioxide is the most universally recognized mosquito attractant and draws mosquitoes from up to 35 meters. When female mosquitoes sense carbon dioxide they usually adopt a zigzagging flight path within the plume to locate its source. Once in the general vicinity of a potential host, other cues predominate, including body odors (sweat, lactic acid, etc.) and heat. Odors produced by skin microflora also play a part in inducing the mosquito to land. Over 350 compounds have been isolated from odors produced by human skin. Either singly or in combination, many of these compounds may be attractants - and many may be repellents. 

As you can see, the situation is complicated and will require many years of testing before it can be sorted out. Visual stimuli, such as movement, also factor into host-seeking. What can be safely stated, though, is that ingestion of garlic, vitamin B-12 and other systemics has been proven in controlled laboratory studies to have no impact on mosquito biting. Conversely, eating bananas did not attract mosquitoes as the myth suggests, but wearing perfumes does. People drinking beer have been shown to be more attractive to mosquitoes. Limburger cheese has also been found to be attractive. Scientists have theorized that this may explain the attractant some mosquitoes find for human feet.