Factual assertions lead to lawsuits, whereas the law generally protects opinion as free speech (but not always -- so be careful). Therefore, you must verify whether your factual assertions are accurate, if you are making any. And don't rely on just any sources. Seek out credible and unbiased sources and keep a record of what you did to investigate the facts included in your post. If you're simply writing an opinion piece, make it very clear that you're doing just that.
Write about Public Figures Or Well-Known Businesses
If you write a post that is critical about an individual or a business, make sure they are very well-known. The law affords "public figures" lesser protection for various reasons. In other words, if a so-called public figure wants to sue you, they're required to show that you made the alleged defamatory statement knowing it was false, or with a reckless disregard for the truth. That's a very high standard to meet. On the other hand, if you wrote about Joe the Plumber (circa 2 weeks ago, when he was a private citizen), he'd have to show that you made the statements negligently--a much easier task.
Admit When You're Wrong
Issue a correction/retraction if you discover that your post contains false information. Don't forget the human component here. Most people will not go to the trouble of suing you if you made an honest mistake and are willing to correct it. It might also be a good idea to request that Google remove all cached copies of the offending post.
Avoid Certain Subjects
You're likely to be slapped with a defamation lawsuit if you write that someone was convicted, punished, etc., of a crime, or that they have contracted a serious disease or are unchaste, if it turns out to be untrue. Also, don't write about something that may negatively affect the reputation of a business. Businesses usually have money and are more likely to hire an attorney to protect their good will and reputation.
Provide a Forum for Opposing Views
There has been an active discussion lately regarding whether bloggers should permit their readers to leave comments on their blogs. Bloggers should permit commentary because it provides a potentially aggrieved party with a forum to voice their concerns about a particular post, instead of in a court room.