Be careful what you write
It's that obvious. You need to be careful what you write, what photos you post, what videos you take part in. The whole shebang. From forums to comments on blogs to social networks. Even anonymously (particularly on forums) you can't be certain that the forum owner won't reveal your identity (or be forced to).
There's no such thing as off the record
When I was a junior press officer I was always told 'there's no such thing as off the record'. And still I made the mistake of giving an unauthorised quote to a journalist. Which got used. And my then boss was not happy. The same rule is true on the internet. There is no such thing as private.
The newspaper frontpage test
I used to work for someone who said "only write something down if you are comfortable with it appearing on the frontpage of the newspaper". This had its drawbacks (no one could give her honest advice) but did protect her and her own staff from wrecking her reputation.
Why am I telling you this - that you already know?
Even the professionals get it wrong
Take this story of a PR professional who Twittered about being in Memphis. He didn't like Memphis - the home of major client FedEx. FedEx didn't appreciate his views.
Here's the story in all its glory: http://shankman.com/be-careful-what-you-post/
And remember, you are never too smart, too experienced or too familiar to ask yourself: does this cast me in a favourable light to someone who I've never met?