First off, this may seem fairly obvious to some, but it's really important to try and ensure that you are leaving comments on blogs that are actually related to that same specific niche market as your blog. For instance, if you have a weight-loss blog, you'll probably find it to be a complete waste of time leaving a comment on a dog training blog.
Also, if a blog in your niche gets a huge amount of visitors each day, chances are the site has a pretty high PR (Page Rank). This makes a great target for commenting on simply because not only can you receive traffic from that blog but you may also increase your own page rank as a result of a premium quality link. If you end up spending your days posting on blogs where nobody is writing comments and it has no page rank at all, you're really not accomplishing anything.
Make an effort to contribute. Most people's blog comments read something like "You have an excellent point here", just to leave a link to their site. This might be enough to get a backlink to your site, but you'll draw far more traffic if you take the time to leave an intelligent comment. If somebody is going through the comments they're going to most likely just glance over yours. However, just by taking a few extra seconds to put a bit of thought into your comment, others may read it and think that you might have some additional thoughts on the subject over on your blog. This means that they will be be far more interested in interacting with you.
There's one more thing that you will need to avoid, and that's doing stupid things like placing a comment on a blog in all capital letters or using 20 exclamation points after your comment. To be seen as an expert in your niche, it's important for you to make sure that your comments are composed correctly using proper English and punctuation.
Finally, while some people might think that posting outrageous or rude comments might be a way to generate some attention, in the end you'll find that this strategy will end up backfiring. In most niches, no one is going to want to reach out to someone who comes across as arrogant and argumentative. In other words, always try to keep it positive!