I use like/dislike in my news reader to tell it what kinds of stories I like or don't like and it seems to do a pretty great job of finding me stuff I want to read.

On sites like this one, I don't cast a lot of votes up or down with one exception. If I post a comment, I upvote every single reply for two reasons. One, I'm happy that somebody chose to reply and two, it helps me keep track of new replies. It might be a misuse of the voting system here, but as far as I can see, there isn't a better way of spotting replies.