Commenting on blogs can be a great way to get some link juice from a high PR site that doesn't have nofollow on. The problem with many marketers are that they are lazy. What being lazy means is that you will most likely have your comments banned and marked as spam.
1. Use a real email address instead of something like: asdfesaf@sadf.com
2. Contribute to the conversation. Saying things like:
"Great article!" or "Terrific work! This is the type of information that should be shared around the web. Shame on the search engines for not positioning this post higher!"
These kind of 1 line canned sentences are totally obvious and they don't even relate in most cases to what was posted. Writing a post or article takes work and it is really an insult to the author to post canned comments.
3. Do not link directly to the product you are promoting. Link instead to one of the blog that provides great content and markets the product.
4. Re-read before you post as you most likely have spelling errors.
5. Comment on other peoples comments to get a conversation going.