My friend started using stackoverflow, but I saw that in his questions he did not up-vote any answers; apparently he didn't realize he was supposed to.
Make it more obvious to new users that they should up-vote good answers (a bigger message about it, maybe? Whatever you have now, it's not working)
Make a bot that automatically posts a comment on every "new" question, e.g. "If you find the answer useful, please vote up the answer"
P.S. I will not explain to him what he should do, so we can experiment on him ;)