Contextual advertising means placing your Internet ads only on pages and websites that mention key phrases (e.g. "lucid dreaming," "out-of-body travel," and hundreds of other words and phrases from this and related fields). Such services are provided by all major search engines, with Google leading the pack. Many social networks also provide contextual advertising.
A key feature is that you only pay for the number of times your ad is shown (or even just the number of times it’s actually clicked on), and you have it shown only where visitors might be potentially interested in it. For example, there’s no sense in publishing an ad for an out-of-body travel seminar on commercial real-estate websites. But if that website has a forum and someone has started up a discussion on lucid dreaming, that’s exactly where your advertisement will be shown. Moreover, if your activities are tied to a town or city, you might choose to have your ads shown only to users who are in that area. For example, if someone from your city visits a webpage about lucid dreaming on a popular esoterica website, then he and only he will be shown your text ad or advertising banner on that page. This clearly allows you ways to optimize your advertising budget. Though quite expensive, contextual advertising is most effective.
The only problem that might arise is an insufficient number of queries on your topic in your region. Out-of-body experiences and lucid dreaming are not very well-known yet, and so not that many people discuss them. This problem is partially solved by diversifying your contextual advertising providers as much as possible. There are usually 3 to 4 large providers on any given market.