everyone uses different way to promote their proxies everyone have their different way to get SEO done and everyone uses different techniques for managing their proxy network
you may promote it by proxy mailing lists or google/yahoo groups, you may do it by getting your link on proxy blogs / proxy lists or proxy topsites to exchange traffic or maybe with the general SEO techniques with onsite and offsite optimization by writing articles, yahoo answers, and more.
when it comes to managing i know two techniques which i have implemented so far.
1) 1st establish 4-5 proxies and when they have steady traffic and income with se, refferal and direct traffic you can move on to work on new proxies
2) keep creating new proxies every few days and keep discarding the old one's