Vote your conscience. That's the best you can do.

I will give ++'s to nodes that:

• Make me think.
• Teach me something.
• Discuss something I haven't considered before.
• Bring clarity to a subject I didn't thoroughly understand.
• Help me out in some way.
• Nodes that show the person writing it has been thinking for him/herself, even if conclusions are not completely refined, and even if it's something I already knew. ...I like to see people learning, it inspires me to keep learning too.
• Make me laugh or smile.
• Are accurate.
• Iterate (or reiterate) something I find myself in agreement with.
• Followups to my nodes with constructive comments, or thank-you comments. I like to know how my ideas can be improved upon, and also like to hear if something I proposed worked.

And on occasion, I'll -- a node that:

• Is a followup by the same person who asked the original question, where the followup shows that (s)he didn't listen to the responses (s)he got.
• Is abusive or inflamatory with no redeeming value.
• Is pure troll-bait or complete electronic-grafiti rubbish (ie, posts to Q&A with nothing but a bunch of garbled meaningless text).
• Sometimes a completely inaccurate response to a question, but only in rare-extreme cases.

I try to get inaccuracies in posts resolved via discussion, and/or Chatterbox /msgs, rather than through voting. I've been making an effort to be increasingly consciensious about handling such things in the least inflamatory way possible, and usually let people know via /msg when I see a minor inaccuracy that they could easily fix in their post.

There are a thousand other cases that could warrant ++'s, and a few others that might warrant a --. I try to refrain from --'s altogether unless something is pretty extremely awful. And I like to give out ++'s as freely as possible. People come back to this site because it provides a positive reinforcement to learning. Kudos (and ++'s) to all those people.