People are blogging more now thatn building websites. Blogs were mostly used by people for online journalism. Now it has emerged into a money making machine.
Most blogs will allow you to leave comments and also allow you to trackback to there website. It's important that you leave comments related blog to yours because there veiwers are similiar to yours.
When commenting on a blog don't just spam there blog site and say "view my site". Not only will that prevent your comment to be approved but there is no value in that comment.
The goal in building credibility to make people want to visit your blog. So make your comment valuable to the blog post and add some useful information that will catch of ther commentors and even the blog author.
A good way to leave a comment would be, write a comment about a product that is talked about on the site. The author has just written a post on the new Canon Camera product. But he left out some key information of the product. You can comment on how good the article is first then mention some key information you felt that was left out from the article.
Commentors will see your comments, they will respect the fact you offered some good information to the site. This will make the commentors come interest in you and find there way to your website.
It's very important you comment to the blogs that resambles your site. Webmasters generate alot of traffic from comment on related blogs. Do it right because it will be a waste of time.