I am an immortal, megalomaniacal supervillain with unlimited resources and patience. I want to slowly eliminate the Earth's axial tilt so that its equatorial plane is coplanar with its orbital plane. How can I achieve this without causing huge numbers of deaths? Assume that I have subjugated the entire planet, and no one will attempt to thwart me. I'm willing to do it slowly over thousands of years so that Earth's life forms have some time to adapt to the lack of seasons.
My first thought is to create some kind of vast, world-spanning magnetically-sensitive rail that will make use of the Earth's magnetic field (or the sun's, or maybe the solar wind?) to create a small asymmetrical rotational force on the planet.
My second idea is to create a space elevator-type structure, with its base on the equator. The tip of the elevator would be a large "sail," comprised of many small panels that could be opened or closed. When closed, the solar wind would push on them, applying a force to them which is transmitted down the elevator stalk to the Earth; when opened, the solar wind would pass through without pushing on them. Thus, the panels would open and close depending on the position of the sail, so that the solar wind would affect it more when it's at a position that decreases Earth's axial tilt, and affect it less when it's at a position that increases Earth's axial tilt.