There are many many ways of advertising proxies, however people only stick to one or two. Because there are so many different proxy networks out there, if you want to stay alive, you need to improvise and get creative. For example, in monetization, the best proxy owners don't make most of their money from some ad network.
1) Toplists - This is a great way for quick traffic. Submitting to sites like Tech-Faq.com and Proxy.org can you give you almost 3k hits in a few days. However, this traffic is temporary and is usually not the best quality. Also, once you submit a site, you can't submit again, so you can't use the same toplist twice.
2) Yahoo! Answers - This will get you the best quality traffic - English speaking. Just do a search for people asking about proxies, and give them a nice list or so. It takes time, but it is worth it in the end. 75% of traffic is US, 20% is UK/Canada, and 5% is other countries.
3) Facebook Pages/MySpace Bulletins - Why not target people who need to access MySpace/Facebook on...well... MySpace and Facebook? Make a page and link to it from your site, and then get people to add it as a fan or something. Then send updates when you get new proxies. Encourage people to tell their friends, which brings me to the next form of advertising.
4) Word of Mouth - This is a very valuable form of advertising. Within my third day of opening a proxy, about 50% of the traffic was direct, and about 60% of that traffic were visitors who were brand new to my site. That means that people were telling other people. For this, you need to have memorable domains like guardtunnel.com. It can even be something like schoolhelper.info, it doesn't really matter. However, domains like 2t74s.com are not very good Word of Mouth domains.
5) Digg/Stumbleupon - Some people use these sites to find proxies. While the traffic is not huge, it is something. Just post your proxy's URL with a little information. This will also get your site indexed and crawled faster.
6) Search Engine Optimization - This is the best long-term form of traffic. People do hundreds of searches every day for terms like 'MySpace proxy' and 'Facebook proxy' and 'unblock MySpace' and so on. If you can get on the 2nd-3rd page, you will get LOTS of traffic. Why do I not say 1st page? Because it is extremely hard, and most people usually skip over the first page since they know the proxies there are probably already blocked.
7) Newsletter Blasts - People, like me, have subscriber lists of people looking for brand new proxies. Some people offer newsletter blasts for free, while other's charge like $1 for them. If you don't want to pay, start your own. You will need to advertise a lot, but eventually, you will be able to sell your own newsletter blasts and mail out your own proxies for free. You can use either a service like Aweber, or take the usual Google/Yahoo Groups approach.
8) Miscellaneous Methods - Like I said, be creative. The most successful proxy owners come up with their own methods. While most of these don't work out, some do, and that's what you're looking for. Track your traffic, test things out. Buy premium listings at a few sites, do some exchanges, figure something out.