Ah, the tanning . . . one of my least favorite parts of competition prep. However, it is necessary. Most competitions having some form of stage lighting that does wash out natural skin color and leave everyone looking odd. Mostly, though, tanning shows that you care. I have seen events where I could predict that final placings based on the quality of the ladies' tans. That does not mean that the tan matters more than the dancing. Attention to grooming details like tanning normally means attention to details in producing the best dancing, so yes, the winners are virtually always impeccably groomed and nicely bronzed.
There are three main ways of getting tanned up for an event: UV exposure, auto- or self-tanning creams, and paints or stains. I am naturally pale and tend to skip from white to red without stopping anywhere in between, so I don't try tanning under the sun or in a tanning bed. That leaves slathering my skin in something. The paints and stains are a quick way to get really dark, so they are a mainstay of Latin dancers in particular. However, they are messy and easy to sweat off, and I dance ballroom and don't need to be mahogany brown. I stick with the auto-bronzers.
All of the auto-bronzers I've used have dihydroacetone, or DHA, as the active ingredient. This stuff turns brown when interacting with skin (particularly dead skin cells, if I remember correctly) over a matter of hours. Because of this, patches of drier skin, like elbows, wrists, hands, and knees, tend to get much darker than the surrounding skin when tanned this way.
I've found that you can even out the color by applying normal moisturizing body lotion to those areas shortly before rubbing on the tanning cream. It seems to prevent the skin from absorbing quite as much tanner, and then as long as I rub in my tanning cream really well, I get much more even coverage.