You are only gonna find about 100 working proxies out of the thousands that you download, and you need to harvest them fresh every morning, then test them, then use them right away.
Filter our the ones with pings higher than you want to deal with too.
For myself, I find that proxies are ONLY necessary for scraping, I have a dynamic IP that changes every time I reset my router, the phoneline is not in my name, and my success rate is more than double without proxies than with (for posting) so I dont use them for posting very often.
For scraping though, Goggle will sick of you sending too many requests form the same IP real quick so use proxies.