1. Pro-actively disclose
Generally speaking it is good practice to tell your employer (or prospective employer) of any activity that is at the periphery of but related to your official activities. For public servants I think that runs the gamut from blogging about related issues on sites like this one, to sharing links with colleagues on Twitter, or participating in discussion groups on Govloop or LinkedIn.
I am of the opinion that proactive disclosure of online activities is less important when those activities are far removed from your professional life. I doubt the powers that oversee the machinery of government care if in addition to your day job you happen to run a local food, exercise, lolcat blog.
Finally, don't go running to the very top of the organization to disclose your activities, simply disclose them to the closest logical individual up the food chain from yourself.
2. Be careful what you link to
This is where I see the most confusion. I've seen a number of people on Twitter link to their department or agency right under a bio that reads "all views my own".
My advice is to not link to any official government website on any of your social media unless you are acting as an official spokesperson for the organization.
Also, and this one is overlooked often, don't provide a link to your social media (e.g. your Twitter) in your official organizational email footer. The implication is that your use of the service is as official as the position you occupy, your phone number and mailing address, when in fact you cannot conduct official business in that channel (especially with external stakeholders).
The same goes for your avatar, don't use anything with government logos in it (like a photo of your ID badge) it conflates you with the department or agency for whom you work.
3. Have a disclaimer, but don't hide behind it
There was a number of blog posts and Twitter chatter in the last few weeks about the subject of disclaimers. Here's my take: a disclaimer doesn't absolve you from being an idiot. Much like you can't claim ignorance if you break the law an online disclaimer isn't a shield. It doesn't protect you or your employer, it only creates a small modicum of distance between the two by telling people that you are not authorized to act as an official spokesperson for your organization.
4. Excuse yourself from conversations that you feel uncomfortable about
During the last election one of the federal political parties responded to one of my tweets about open data. The conversation quickly turned partisan. My reaction was to simply tweet that given my role and responsibilities as a professional and non-partisan public servant I was unwilling to engage any further; an explanation that was readily accepted by the party in question.
If you are pressed even after you excuse yourself, just stay silent, let anyone watching it play out draw their own conclusions. I
5. Don't bitch about work
Venting may feel good now, and a status update is so easy that its often we get caught up in the moment, but there aren't any private places online where you can complain about your employer.
Good rule of thumb: don't say anything online that you don't have the wherewithal to say in person because you will be held accountable by someone somewhere down the line.
Like mom said, it's always better to be safe than sorry.