Simple solutions that show up the flaws in highly sophisticated technology are awesome.
(Remember from a major diamond heist how spraying an infrared motion sensor with hair spray can make it invisibly stop working?)
This article suggests that you might be able to sneak something past backscatter X-ray machines by slathering your skin with skin cream or sunblock containing zinc oxide or titanium dioxide. The idea is that ZnO2 or TiO2 don’t reflect x-rays very well — just like metals. (Organic materials, like your skin, normally reflect quite well.) By slathering yourself with sunscreen under your clothes, you could create natural-looking splotches of dark on the operators’ screen in order to conceal the outline of contraband.
Titanium dioxide based sunscreens might work against millimeter wave technology as well (allowing you to create bright areas that hide the bright outline of contraband). The only difficulty is that the TiO2 has to be a “substantial fraction of a wavelength” thick, where a wavelength is measured in millimeters.
I would suggest someone start manufacturing TiO2-impregnated clothing, but it appears someone’s beat me to the punch and (linked from the article) “radiation protective underwear” is already on the market.
“Unlike conventional transmission X-ray, which passes X-rays through an object to image its interior, backscatter X-ray scanners use low power X-rays to illuminate your body and catch the reflected energy to generate an image. The lower the atomic number (or “Z”), the stronger the reflection. Organic matter, like your body, is made of relatively low Z matter like carbon, oxygen, hydrogen, and nitrogen. This strongly reflects the X-rays yielding a brighter image. Lithium would also be a strong reflector. High Z materials, like most metals, will reflect poorly, showing up as dark splotches. Certain skin creams, like zinc oxide (used in sunscreen or baby diaper rash treatment), or titanium dioxide (also used in sunscreen) should reflect poorly. Iodine would also tend to be fairly absorptive. Lead should be particularly dark in an image.”