I vote up questions that are good questions -- they're not covered in the FAQ, they haven't been asked before recently, and they show that the poster has attempted to understand the issue before posting.
I vote up answers that are technically correct (as far as I can tell), that contain more information or a different approach, or that point the original poster to additional information.
I vote down posts that are personal attacks or blanks (if no one else has set them to -1 yet).
I hit Worst Nodes daily... I'll bring mediocre stuff up to -1 or 0 if necessary (especially if it's a registered user -- no sense in driving people off).
Posts that are technically incorrect or need more explanation deserve encouragement, not --, so I usually send a message to the poster offering unsolicited advice.
Those are my rules, and I like them.