I thought up a huge theoretical framework for considering memetics as celestial bodies, with gravity and spin and light-speed and all that jazz. I even worked out a little bit of half-advanced waves/particles, but I decided that really wasn't going to help me any.
In the end, however, there were few things that really popped up as a useful:
Some things are big - black holes and stars with considerable "memetic gravity". For example, you might consider BoingBoing, health care, or your office as having a truly impressive gravity field (each person builds their own "star map"). Some things are more moderately sized. This site, for example, has noticeable gravity - perhaps comparable to one of Saturn's smaller moons. However, similar to a comet, few visitors get close enough to get slowed by the gravity, let alone pulled into an orbit.
Other things - the most common type of things - are really tiny. Dust, asteroids. Ads, for example. Their purpose is to tug on a user until his vector is headed in a direction more to your liking. They utilize high proximity and careful spin to maximize their gravitic pull. Actually HITTING a user dead-on is nearly impossible, so it's best to think in terms of gravity, rather than physical impacts.
Of course, celestial bodies both big and small don't pull in one direction. So if the user comes in from a direction you didn't expect, you can easily end up slingshotting them the wrong way. Fortunately, most users are orbiting in the plane of a larger celestial body when they encounter your little celestial bodies, so you know roughly their vector.
But if you guess wrong, you can do worse than have no effect: you can push them away. Throw them somewhere further from you. Like if you advertised Windows on SourceForge. Like if you put up any Republican-type posters anywhere in Seattle. These things do pass very close to the passerby - but the vector of the passerby is wrong, and the slingshot hurls them away from you.
Anyhow, the key to this kind of thinking is to think in terms of orbits. Your ads will be placed in orbit around the same things the users are orbiting, and your job is to pull them away. You need to have a very similar orbit, a lot of spin, and a carefully placed storm of ads.
You need to be delicate. Place it wrong and you'll end up flinging the audience deeper into the heart of the system, or out into the depths of space - certainly not at you.
Of course, without the conceptual framework identifying memetic orbits, orbital patterns, and measurement of the audience's relative distances, you can't really usefully use this. Plus, each such "orbital framework" is built in the mind of each user, so you're operating at one step removed...
But the idea that you can think of ads as tiny gravity wells placed in the orbital path of users might be helpful when you think of marketing.