Be nice, (that first because some don't have it in them),
be yourself (if you are naturally a grumpy old man see number 1 and try to be nice).
If you read all the way through the post leave a thoughtful comment, not just thank you Nice post, you can do that every now and then, but in the long run when the blogger looks back over your comments, you don't want them to see a trend of that type of comment.