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 to Mr.Wizard's question. That set my question up-voting policy because a good question is worth an up vote.