What can I wear to avoid getting frisked and crotch searched by TSA at the airport?

Wear trousers without metal and by preference ones that stay up without a belt. It is not a given that you will not be checked but it will make it less likely and you will feel better when you are standing there.
Take everything out your pockets, including hankies and odd pieces of paper. You can put them in your bag or a coat you take off and send through the machine.

And most important, do not worry when you need a pat down (not even when they feel close to those parts of you where you'd rather not have hands.) They do not mean anything but making sure that everybody is safe in the planes. You surely are not the only one that is patted down.
This may sound not helpful, but the way you stand makes an impression on the officer doing the patting and being self assured and feeling you can look him/her in the eye makes a different impression and makes it less likely they feel the need to search your crotch area.

I am a woman in my fifties and dress in activity trousers going when flying, no metal but a few double layers and here there are the 'look through' machines more often than not. Over the last 5 years I have been patted down so often that I am now surprised if it is not done. This is because of the machines not being able to 'look through' those trousers but I am not willing to change as they are the perfect choice for my travels otherwise. I take the patting, (although it seems that the machines are getting better at handling them.)
I never trigger the metal scanners on the airports that use those.

Make sure you feel confident, look the officer in the face and let them do their work.
It is not against you. You are just the person where the machine beeped. Those machines are often beeping where nothing is found by the patting.

You want clothing that appears invisible to the millimeter wave scanner, so you appear naked i.e. no false positives. Any metal will obviously show up, but anomalies in fabric density can also appear as a suspicious object. When I travel I avoid:

• Velcro fasteners,

• Cargo pockets (multiple layers),

• Tucking in my shirt (rumples),

• Baggy clothing or undergarments,

• Shirts with thick collars, and

• Sweating or wearing wet clothes.

At least once for each of the above, the scanner instructed the blue-shirt to pat me down in the relevant area.

Also, to avoid hassles with security, don’t wear:

• A toupee with cocaine.

• A shirt with Arabic writing or a Bitcoin logo.

• Everything you have to avoid a checked bag fee.

• Prototyped wearable electronics.

• A penis if you dress as a woman.

• Nothing.

This will not help you this week, but if you're eligible to do so, you could sign up for TSA Pre? (or another program like Global Entry, if you travel internationally, that includes the same benefits).

Precheck lanes at most airports have regular metal detectors, not millimeter wave scanners, so they're unaffected by how your pants fit. There is no guarantee — you could be chosen for a patdown search or sent through the millimeter wave scanners for a further check after setting off the metal detector or randomly — but it's much less likely to be an issue as long as you remove all metal.

I used to have the same problem, needing secondary screening about 60% of the time. Now, after I enter the scanner but before I raise my arms, I pull my pants up and forwards, so at least there's contact with my hips and lower back. Usually it'll hold there long enough for the scan to complete. Since I've been doing that I've never had any secondary screening. That said, this probably won't work for all body shapes.

Wear a kilt.

The last two times I've flown, I wore a kilt, and (even after being frisked) could have smuggled an inflated beach ball through security.

Don't wear underwire bras

Okay, maybe not very useful for you per se, but for other people. I tend to get frisked due to underwires that are likely somewhat larger than on average female traveller.

Personally, I don't think there is fireproof way to avoid patdowns though, so I don't have better answer for what to wear.

While not a frequent traveler, I have gone through airport security at least 20 times with a LOT of stuff in and attached to the outside of my bags that I would think would warrant additional screening. However, I dress very... hoboish, and have never needed a patdown.

What I mean by that is sweatpants, a single plain white undershirt, and a plain weather appropriate overshirt (which I usually send through the belt scanner). If it is a very early flight, I will actually wear sleepwear pants instead for comfort on the plane. The pants are indeed baggy, but low density poly/cotton blend so they are basically transparent to the scanner. There is simply nothing there to show up as something unusual. Jeans are what I would call a high density fabric, especially around the zipper and belt areas.

Additionally, wearing minimal clothing tends to prevent me from sweating, which is a red flag in a security line. I would rather be cold for a few minutes than pulled aside for looking suspicious.

I have had by bags checked a few times, the funniest time was taking a block of fudge back home, according to the scanner tech it look like a block of plastic explosive. Only pro-tip I can give inside the scanner is to keep the legs and arms spread as wide as you can, so nothing is bunched up. I have observed the people in front of me, and those that do not use the correct stance sometimes need to get scanned again, or patted down.

Unfortunately, sometimes, you're just out of luck. I fly often enough and I've definitely had unpleasant experiences, including being touched, um, yeah, there. Under my underwear (if you ask me that would be sexual assault in any other scenario). But generally, I've managed to keep those experiences to a minimum by following certain rules that I've conjured from experience:

• Take off everything other than your pants, your socks and your shirt. That way you generally won't set off anything. That also means: Empty your pockets (entirely), and take off your belt, shoes, sweater and anything else you may have on top. Even if they say you don't have to. I've been told I don't have to take off my shoes, only to have something detected most likely because of my shoes, leading to a pat down.

• Dress well, even if you're on a casual vacation. It doesn't have to be a suit if you're a man (although a suit never does any harm), but wear dress pants, dress shoes and a good looking shirt. For women, something similar should probably do (although I've almost exclusively been flying alone lately so I don't have too much experience in that department). You want to give off the impression that you are cooperative and friendly with airport security, but if they do something that isn't acceptable you won't hesitate to hire a lawyer and sue their ass. More on that later.

• Make sure to look well groomed. If you're a man, shave, I don't care how much you love your beard. I also like growing my beard out, but when I fly, if possible, I shave. Unfortunately, beards are associated to a certain extent with terrorists for some people. Make sure your hair also looks neat and professional.

• Don't mess up anything while putting your stuff in the scanner. Take your time, if necessary, to do everything properly. Show your empty water bottle without them having to ask you. Take your laptop out and put it in a separate bin right away. Don't worry about the people behind you, do things thoroughly. Even if your previous flight was late and you need to catch a connecting flight - that will take less time than a potential patdown and God knows what else.

• Be friendly (assuming they don't start doing anything weird). And not snobby-friendly, genuinely friendly. Greet them (good morning, good evening, whatever), don't forget your please and thank you. Essentially, you want to gain their respect and trust, and courtesy definitely goes a long way there.

To sum it up, you want to make their jobs easy, prevent anything from popping up on the scanner, and gain their respect in that short period of time through the way you present yourself, both in a manner of being courteous but also by looking like someone who, as I mentioned earlier, won't hesitate to hire a lawyer if they do something that's uncalled for, regardless of if you can afford a lawyer or not (but to emphasize: look that way, don't try to pull off an act out of it). That has definitely helped me along the way. It's unfortunate that this is where we've arrived, but unfortunately, I often don't have any other choice than to fly, and I am guessing you're in the same boat (actually, for that matter, I would much rather be on a boat than on a plane... Oh well).