I've made a number of observations on this, personally, and over the years have figured out some things that help them avoid me (or prefer others):
* High vitamin D. You're either getting this from sunlight secondarily or foods (eggs, fish, etc.)
* Garlic. Lots of garlic. This one's the biggest, and for me one of the easiest to address since it tastes good with food.
* No perfumes or scented deodorants. They love that shit. Avoid it like the plague.
* Generally low-carb diet (no crap starches).
* Sulfur. This one surprised me, but it's something we naturally need and don't usually get enough of in our diets. I found it out on a lark during a particularly buggy camping trip when a woman I was with (who wasn't being bothered as much, contrary to my experience that women are bothered more often) swore by it.
My wife also noticed a marked decrease in skeeter interest in her when she stopped being vegan.
Also anecdotally and related, because i know this comes up a lot in outdoor talks lately: ticks. They hate oregano and garlic oil. And neem oil, topically, mixed with lotion. I've had them climb up my sock and then fall off (seemingly intentionally) repeatedly when trying to get onto my skin after a diet heavy in garlic and oregano. It's at least as effective as DEET and sticks around longer.
Sweat also seems to attract them, perhaps due to it causing the odors to be more airborne.