But every time you prestige again they don't get wiped clean.
(I assume you're referring to the glowing green challenges). The only prestige challenges that get wiped clean after prestiging again is every one below Spectre in this image: http://screenshot.xfire.com/screenshot/natural/8efc09ba7a7d490614a5b2e2342a271ee6bb4ee6.png
Every other prestige challenge (green glow) is cumulative and adds up over EVERY prestige level without ever getting reset.
Hope that helps.