A few good point have already been mentioned, making sure not to spam, making sure that blogs are of high quality, and of course, the dofollows and the nofollows. Saying more about that would be overkill methinks.
Have a look at it from another perspective, now that the SEO factor has been taken care of
Commenting on blogs can help you gain a good reputation as well, and this can also help bring more people to your blog. Simply by reading the comments you have made on other blogs. Reading through a post before you comment, asking questions about the post shows bloggers that you have read what they have written. what this does (well it does with me
) is tug at their curiosity and they would want to know who you are, and they would follow your link to your blog or site, and more than likely read something you wrote and comment on it as well. (I've tried this, and it works). If your blog is of a high standard, it could get the ball rolling and have many people coming to your blog and looking at what you wrote.
If you comment though, if its like wordpress, and it gives you an area to add your website, add it there, and not all over your comment. Its that, coupled with a badly written comment, that generally has a comment ending up on the spam heap.