Question: Mine is simple. I up vote any answer I think will be of value to the OP and to others with a similar problem. If I think the answer will only be of use to the OP (this usually stems from the question being very localized), I will up vote the answer when the answer takes a broad view of the question and goes sufficiently beyond the local needs of the OP to have archival value. I always feel sad when such an answer is lost because the question gets closed.

To be fair to all who answer a question, my policy requires me to revisit a question whenever a new answer has been added to up vote again if another worthy answer has been added. I accept this responsibility.

I'm pretty sure some others have different policies. I don't claim mine is best. In fact, I suspect it might be too simplistic. I'd like to see other policies expressed. One might be so much better than mine that I will replace my current policy with it.

Do you have a different policy? If so please write it up as an answer.

### Edit

I already discussed my criterion for rating a question as good in [this answer](https://mathematica.meta.stackexchange.com/a/1308/3066) to Mr.Wizard's question. That set my ***question*** up-voting policy because a good question is worth an up vote.

Answer: 
I wasn't around when this was asked, but I feel I should also be putting in my take.

My upvoting criterion is capriciously arbitrary: if I "enjoyed" reading the answer, or learned something genuinely new, or it is an answer that I would have written myself (also counting the case where it was written in a far better style than I would've done), only then do I upvote the answer. It is the exact same voting criterion I use on other SE sites, and I like to think it has served me well.

My downvoting criterion for answers is way more strict: I will only downvote if an answer is actively wrong/harmful, and the answerer has proven uncooperative in nudges (through comments) to fix his/her answer.
My approach is subjective. I upvote answers from which I learn something new and unexpected. I sometimes downvote an answer when I have the impression that it too heavily relies on comments or answers already given. Sometimes I downvote questions if the OP doesn't participate ("Do my homework but don't bother me with questions or comments"). Sometimes I regret that one cannot upvote comments.