There are some classic approaches to media buying, too. For example, banners on the websites that your target audiences visit frequently are well suited to drive last-minute purchases -- for airline tickets, weekend vacations, impulse items and the like. A clear message and a short-term offer can interact with a prospect's well-established desire to trigger an immediate purchase decision.

Buying search results on relevant terms is a good way to drive specific actions. The difficulty is that at some point you'll have reached most of the people who are making that particular search, and from then on you'll start to experience diminishing returns, cutting into the efficiency of your buys.

WOM tends to be most important for restaurants, hotels, luxury brands and other products or services where reputation and expectation are important elements within a prospect's consideration set. Campaigns for these brands and products should therefore seek to generate and build on WOM at least as much as they should seek to drive clickthroughs and conversions.

For entertainment properties like movies and TV, online video samples can be most compelling because they actually give the target audience a chance to sample the finished product.

Advertisers aiming for a certain level of authenticity should look to move their media buys toward the far end of the long tail. This is because the larger sites tend to attract a very large but general audience, compared with extreme niche sites like Rotten Tomatoes, which attract more hardcore and knowledgeable segments. If you know who is sitting at the center of your bullseye, you can look for the sites with the "pure voices" that attract and hold the attention of those segments. Not only does the web contain specific sites that specialize in attracting the more specialized audiences for almost any advertising strategy, but prominent ad networks -- horizontal and vertical -- have inked deals with many of these sites and can help you grab their avails at reasonable prices.