Okay, this works:
First of all, there are myriad myths about these little buggers. They don't burrow, and any chiggers on your skin will be washed away by a vigorous bath...so the nail polish-type remedies are likely placebo in effect.
As many have mentioned, DEET is the pre-eminent solution. It WILL keep them away. Other repellants, such as sulpher 'may' be effective. I haven't tried them. Tucking pants into socks, wearing light-colored clothing etc, all help. Avoiding tall grass, moist areas and so on...all good ideas.
What if you've found yourself in a likely area without some form of protection? Chiggers are far more fragile than their agonizing after-effects would suggest. The key to defence though, is that they don't bite immediately. Some sources indicate they may take 1/2 hour or more to dig in and feast. The longer they are left in place, the worse the symptoms.
So, simply RUB them off. A vigorous rub-down of areas likely to be infested (legs, arms, etc) is very effective. Every 20 minutes or so, briskly rub those areas known to have contacted vegetation, but especially feet, legs and arms. The ones you don't kill (most) will be rubbed off. Whever I remember to do this, I don't get bitten...or get very few bites.
As soon as possible, bathe; soap and hot water. You'll kill or wash away any left on your body...hopefully before they've had lunch (you).
Too late: You feel the itch, see the bumps? Bathe anyway. It takes hours for the symptoms to appear, so you likely won't kill any of them. They're gone...but you might get a few.
The real value is the heat...we're talking histomines, the compounds in the skin that transmit the feeling of itching. Heat activates them. Your body can only produce so much histamine over a period of time, so a REALLY hot shower depletes the histamine level and will give a period of relief.
Then take a couple of benadryl (ANTI-histamine...get it?). Finally, rub some topical creme over the affected area...lots of it. Rub hard! It's almost like scratching, but beneficial. I've found anything containing hydrocortisone to be effective, but others prefer dedicated anti-itch products (which contain anaesthetics). The hydrocortisone is more effective in reducing the swelling.
Go to sleep! (Yes, you'll be able to...). The affected areas will be tender--after all, you've been punctured and injected with a foreign substance, maybe hundreds of times--and may itch a bit, but it will be bearable. When you get up, take another benadryl.
I got an infestation that was one big, raw, hamburger-looking welt from my knees to my armpits--much more severe than any picture I've ever seen. It took a while, but daily HC creme, and benadryl morning & night made it possible to live a normal life in that condition. If the itch got bad, I simply jumped in the shower and parbroiled myself again.
(I like hot showers, but I'm talking really hot. Turn the water as hot as you can stand it, play the stream over the affected area...then turn it up some more...it's much better than scratching.)
Okay, this works: