Paul Taubman is running the Ultimate Blog Challenge. As the administrator he gets to read lots and lots of blog comments on his and other's blog posts. He has identified some poor comments and the type of comments that get you noticed.
The bottom line to leaving comments is that you want to try to leave information that is helpful and beneficial to the other readers (as well as to the blog owner). Offer good advice to those that come after you. Remember to be friendly and approachable!
Learn which comments you should avoid and which are welcome. Read: Tips To Leaving Comments.