I would like to echo a post that Scott Morrison made on Tex.SE. A link to the post here, and the text of the post:
"I'm a moderator from MathOverflow, and this "question" is actually unsolicited advice, based on our experience from the initial launch of MathOverflow.
We should encourage everyone to vote positively as often as possible!
Every Stack Exchange site will eventually end up with a different "base level" of voting --- that is, the expected number of upvotes for a question of a given level of excellence. (This effect occurs because people see a good question, but already with a certain number of votes, and think "oh, I would have upvoted this, but it already has enough".)
It's easy for us to affect this "base level" by encouraging high levels of upvoting now. We're setting the standards, and this really will have an effect.
(On MathOverflow, we were very active about this early on, specifically encouraging all the initial round of users to vote early and often. You can compare statistics, and see that the average vote total for a MathOverflow question is much higher than on any of the other SE 1.0 sites.)
In case it's not obvious: the rationale for wanting this base level to be high is that it provides better positive feedback to good contributors."
Note that the number of high reputation users will be one of the measures that SE will consider in deciding to move the site from beta to a full site, a higher average vote will make it easier. Also it encourages people to post and answer questions because they see the positive feedback for their effort. So when you see a good question or answer upvote it.
Based on our experience on cstheory, I would also add the following recommendation:
when voting, don't consider the total number of votes a post already has, your vote should be independent from other users' votes and the total number of votes, if you think a question/answer is good/interesting/useful/helpful upvote it.
The reason for this is that I have seen users trying to fix the total number of votes and therefore don't upvote posts even when they like it because they think it does not deserve that many votes, which keeps the average number of votes quite low.