NOTE: this is not site policy, just my personal suggestion
Being a newcomer and having your post downvoted can be very discouraging. This isn't necessarily a bad thing—obviously we want to discourage people from posting things that are not worth our time to read—but it doesn't provide much feedback other than "something about this post/comment/question/answer makes it undesirable to have on LessWrong". So here's my idea:
If you downvote something that isn't obvious spam, you should comment the reason why. This will nudge the newcomer in the direction of "people don't like this particular quality of my post/comment/question/answer" rather than "people don't like my post/comment/question/answer." So as to avoid flooding bad content with comments, simply upvote any comment that states a reason for the undesirability of the post/comment/question/answer instead. Hopefully this helps the newcomer get feedback about what they should change, rather than just blindly guessing.
I don't think such considerations apply to upvotes nearly as much if at all. Upvotes indicate agreement or approval, which doesn't need to be explained as thoroughly as disagreement (which usually involves having separate, alternative ideas in your head different from the ideas of the one you are disagreeing with)