- Say something relevant to the post.
- Do not blatantly promote yourself. Interesting comment will make people investigate you, and that is better than any commercial you could write.
- Only put links in the comment if they are highly relevant. Bloggers do not like visitors to see too many exit signs.
- Try to write in a voice and manner that matches the blog. If it is a political blog bashing someone, it is fine to leave similar comments if that is what you want to say. In this blog I would be a bit puzzled if you said "Buzz Sucks!"
There are more things I could say on this topic, but there are exceptions to rules based on the blog. Keep in mind that when you are posting on a blog you are a guest in the blogger's house, but the blogger probably likes a lively debate.