In the above video Matt Cutts, Googles head of webspam answers the question “How can I guest blog without it appearing as if I paid for links?”
1. Keep it on topic.
2. Avoid keyword rich anchor text.
3. Provide interesting opinions
4. Don’t spin, write unique content for each blog
5. Do it in moderation
6. Paragraph about the guest blogger, who they are, why they were invited to blog. Maybe link to the guest bloggers G+ profile using the rel=author markup.