Ever since Google released the Penguin update, guest posting has enjoyed a huge increase in popularity. When properly employed, guest posting can be a very powerful tactic. No other strategy is as ethically justifiable, or as compliant with Google's T.O.S as the creation of great quality content.
Getting Started
The first step in any guest posting campaign should be to identify which blogs in you want to target. Look for blogs in your niche that have high editorial standards. Read through a few of their recent posts and assess the quality of their content. If any of the blogs appear to have low editorial standards, remove them from the list. Don't focus too heavily on Page Rank, it's, Alexa rank can be a useful way of assessing the visibility of a blog, and domain authority will provide a clearer picture of a blogs true authority than Page Rank.
Once you have identified a few blogs that are deserving of the great content you intend to write, get involved. Comment on their posts, follow them on Google+ and work on building a relationship. Linking to one of their posts from your own blog, and then letting them know you have done so by leaving a comment on the linked post can be a great way to break the ice. The chances are that they will click through from your comment and read your article, so they already know that you write great content before you have even asked them about the possibility of becoming a guest author.
The Pitch
I much prefer to ask about the possibility of guest posting via a Google+ hangout than an email, but if you do have to resort to email requests, then make sure you tailor the request to the blog owner in person. Take the time to find out their name, what aspects of your vertical they write about, and provide information in the email that lets the recipient know that read their blog, and know what they are about. Propose writing a post on a specific subject that you think will be well received on their blog and provide previous examples of your work, either on your own blog or as a guest author elsewhere. It's important to remember that the owners of popular blogs are inundated with guest post requests on a daily basis, to even stand a chance of success your pitch needs to be right.
Why Quality Is Imperative
Guest posting is now the defacto link acquisition tactic of choice for many internet marketers, and just as standards amongst individual internet marketers vary, so too does the quality of the blogs that they target. There is no shortage of blogs that are happy to accept poorly written three hundred word articles, but that does not mean you should use them. Google made their stance on guest posting for links transparently clear in a web master video that was released way back in Oct 2012. In this video, Matt Cutts stated that links associated with high quality and well written guest posts are precisely the ones that Google want to count. Matt also mentions that Google are not interested in counting links from low quality guest posts, that are written to a "bare minimum" word count and exist solely to provide a link to the author's site. Many SEO's have already taken this as an early warning of things to come, and predict a future algorithm update that will penalize low quality guest posts.
Leverage Social Media
Linking to your social profiles in every guest post also provides an additional benefit, each link increases the authority of your post pages. This means that any articles you share via Google+ and other platforms will receive more link juice.
Engage
Don't consider the publishing of your guest post as the end of the line, subscribe to comments on your post and engage your readers. Answering any questions your readers have and responding to feedback is also means you gain an additional link every time you reply.
Remember that your readers will likely include other web masters in your vertical, I have gained a number of great guest posting opportunities through nothing more than engaging with readers in this manner.