I don't comment on blogs a whole lot. Most of the time I haven't anything to say. However, when I do decide to leave a comment, I think first how I should write it.
Blog comments should be as short as possible while still driving your point home, just like pretty much any type of writing. Generally a compliment is in order, if you so choose, followed by a statement or question regarding the post. Comments can be warm, friendly, and informal. I'm not sure about others, but I really appreciate comments when I can get them, whatever the content may be, although personable comments are the best.
Really thinking about it, I should probably leave more comments. Maybe I'd recieve more if I did...Anyways, happy blogging!