I think the best token of appreciation is a “vote up” – please do vote, it helps to promote good content on the Community. Private messaging is on our roadmap for this site; we just don't have it yet. I think general forum/community rule of thumb is not to pollute threads with too many "thank you" messages. And I agree with that. But I also don't see big harm if you do say "thanks" once in a while especially if you add to it one or two sentences how far you were able to get with the help of the thanked person. But once again, I think general notion is to assume that everyone appreciates everyone's effort and time and to minimize all sorts of single-word posts like "thanks!", "great!", etc.