I upvote if I think the post is contributing to the current conversation, and strong upvote if I think the post will contribute to future and ongoing conversations (IE, its' a comment or post that people should see when browsing the site, aka Stock vs. Flow).

Occasionally, I'll strong upvote/downvote strategically to get a comment more in line with what I think it "deserves", trying to correct a perceived bias of other votes.

I think I upvote mostly like this (I'll edit this answer if I remember more reasons):

Strong upvote: correct (if applicable) and important.

Upvote: correct (if applicable) and slightly important, or not completely correct but interesting/potentially important (In this case I usually also reply). I also tend to upvote comments under my own posts much more because I feel the need to thank the person who took the time to write the comment somehow.

I don't always follow these rules exactly, but mostly. For example, sometimes I upvote to close the gap between two comments I consider equally important under a post.

I vote based on how much I think something contributes to the discussion, aiming for a roughly equal split between upvotes and strong-upvotes (which I expect to be somewhat of a Schelling point). Same logic for downvotes vs strong-downvotes, though I obviously don't split 50/50 between upvotes and downvotes.

Karma should be awarded when a post or comment is high-quality, especially when it's hard for others to notice the high quality. So I strong-upvote a comment when it has an important non-obvious insight and I needed to think for a while before understanding it.