You can politely ask rationalist questions when someone says something irrational. Don't forget to smile!

You can write letters to the editor of your local newspaper to correct faulty reasoning.

You can visit random blogs, find an error in reasoning, offer a polite correction, and link back to a few relevant Less Wrong posts.

I don't believe these are seeds, such actions don't leave lasting impression that grows under its own power. A lot of energy can be spent in vain correcting specific errors in people who won't take a hint. It might be much more effective to focus on educating people who can actually be expected to make rationality one of the guiding principles in their lives, learning more themselves than an occasional correction by others allows, and some of whom would spend energy propagating the meme.

A textbook on rationality, a rationality seminar, or advertising thereof would be seeds, but probably not arguing with random people who are wrong.