Solid advice. It's also a good idea to personalize the letter when contacting the site admin. Make it clear that you are writing an article specifically for the given blog and not just looking for a place to upload linked content. Cheers.
Keep track of competent blogs in your niche and follow their posts in the comments section below them. Engage with other users and the article writers. Gradually you will be start getting noticed and you will build a relationship with the authors and blog owners as well. Then approach them with a request for a guest post with a link back to a related page on your own site. This is the way to be good at guest posting and to continue this work for times to come. Do not just mail them in the first instance requesting a guest post. How do they know you or your credentials?