I roughly follow the following (prioritized) rules:
Up-vote if I want to see more posts like this/down-vote if I don't want to see more posts like this, regardless of the current total.
A comment that I do not feel very strongly about I may up- or down-vote based on what total karma I expect the comment of this kind to deserve.
Very occasionally, I might like or dislike the author for unrelated reasons, and decide to up-/down-vote based on that.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
(I plan to make these threads from now on. Downvote if you disapprove. If I miss one, feel free to do it yourself.)