I agree with I appreciate a thank-you, particularly if it gives a little explanation of how the final solution turned out.

If it doesn't just say "thanks", but adds content, that is great. If everyone said "thanks" it would double the number of posts!

I've sometimes sent private message with a specific thanks.

I suppose adding a P.S. to the original post is a good idea, too.

And of course, voting up the helpful node is an indication of thanks!

That makes me think it might be an interesting feature for a design of a new forum: have a button to flag thanks, adding a one-line comment. It can be appended to the post in a succinct way, without all the screen space of a whole new node, and if not public at least ack's the poster that it was read.