I upvote it for a day or so, then later return to downvote it. This gives the impression that two objective observers who read the thread later agreed with me.

This strategy can be eliminated by showing a count of both upvotes and downvotes, a change which has been requested for a variety of other reasons. I imagine it solves a lot of problems of anonymity, but it makes Wei Dai's dilemma worse. It makes downvoting the -1 preferable to upvoting it.