I upvote posts for a few reasons, as follows:

• It is cute/funny/witty/clever/fun
• It's well written
• It's informative
• It teaches me something about Perl
• I just feel like ++ it.

Unless I find a post that really pisses me off, or seriously deviates from the Perlmonks feel, I never --.

Those are my voting habits, use them to your (dis)advantage!