I try to always upvote a pertinent comment, and I try to respond to that comment. A vote is after all just a vote, I can go and upvote a lot of comments or blogs or post, but if I say nothing, what does the person think about that vote? I know I appreciate the vote, but I also think to myself, "it would have been nice if they said why they up-voted me". So I try to say why. I also (as a means to say thank you) try help people find similar content if they enjoyed my post, or comment.