Guest blogging 101 comes down to this.

Find a few bloggers whose content you think you could match, and then I do a brief brainstorm. Jot down an introduction and then send them an email.

Mine looks like this: "Hi, I have written this Twitter Tip/Social Media guest post and was wondering if you might be interested in taking a look."

After I receive a "sure, send it over," knowing that I haven't actually finished the post, I buckle down and finish it. I usually reply with "yes, just wrapping it up now," which is what I'll do.
Usually, it triggers me to finish the post lightning fast since I know there will be a definite return from putting forth the effort.

When you're ready to email the bigger sites directly, find the contributions editor and then dropping them a note. This I found works very well after you already have some experience.

Attached is an actual email pitch that I have used to get guest posts written up.