There have been discussions of this before. I think the reason why we don't have anything like official guidelines is that most of what one can say about how to vote is either obvious or controversial.

Here are my own principles:

• Vote things up when they are particularly good (i.e., LW would be improved by having more things like them) and down when they are paricularly bad (i.e., LW would be improved by having fewer things like them).
• Vote sparingly (think of my opinion as quality signal plus random noise, the noise depending on my own personality quirks, taste in writing style, etc.; vote only when the signal is significantly above the noise).
• Vote to correct badly underrated/overrated things, but with even more circumspection (if this is done often, either the rating something ends up with depends strongly on the order in which different people see it or else you get multiple people revisiting each thing and changing their votes endlessly).
• In cases of mere disagreement with a generally reasonable other party, prefer vocal disagreement to downvoting. Downvoting is better suited to cases where persuasion is unlikely to succeed.
• Be especially reluctant to downvote things on account of political disagreement.
• Vote according to the (de)merits of the thing being voted on, not its author.
• For good or ill, many people find being downvoted personally hurtful (more so, I think, when their comment or post is rated negatively afterwards; going from 0 to -1 hurts more than going from +2 to +1). Accordingly, be just slightly more reluctant to downvote than to upvote at a given level of (positive or negative) quality.

I think almost everyone would agree with the first principle. The second and third seem obviously debatable, and in particular I know that some people think one should never take earlier votes into account when deciding what to do with a post or comment. The fourth and fifth seem more often agreed with than disagreed with in public, but it seems clear that not everyone acts that way and politically charged topics seem to attract more opinion-based voting. Almost everyone agrees with the sixth. I'm not even sure whether I agree with the seventh.