"Loose lips sink ships," they used to say. Now, sinking ships are not the worry they were in WWII (in fact, the sinking of ships may not necessarily be a bad thing from our perspective). Now, the worry, at least for our purposes here, is going to jail. Standards and cultures of communication have evolved to maintain a different kind of security. It is still a bad idea to talk openly about actual operational details of clandestine actions, but what is at stake is different. WWII soldiers and commanders were not so much concerned with having the book thrown at them by the enemy, they were concerned with carrying their mission through to its completion and not having their ships sunk in the process.
Taking action on enemy soil, which is what our current situation amounts to in this country, entails a whole different set of concerns. You don't even have to carry out an action, successfully or not, to go to jail for it. Felony conspiracy charges will hold if the prosecution can simply prove that you talked about doing something illegal and took even the smallest step toward accomplishing that illegal action, even if neither act, in and of itself, was illegal. For instance, to be convicted of "conspiracy to jay walk", all you have to do is discuss plans to jay walk with a couple buddies (or even just one buddy) and take one step toward the opposite side of the street. You don't even have to step into the street against a red light. Either act itself is legal, but together, they are considered felony conspiracy. (By the way, this example wasn't purely illustrative—people have actually been charged with conspiracy to jay walk during street demonstrations.)
So, make sure that the people you do talk to about clandestine actions are held in the utmost regard and trust, and that it is in some way necessary for them to hear what you have to say in order to accomplish what you're after. Again, another military aphorism is useful here: information is shared on a "need to know basis." If they don't need to know, you don't need to (and shouldn't) tell them.
Of course, after the action has been completed—successfully or not—there is no need to tell anyone about it. If you feel you need to get the word out about the action, make sure that this is done in a carefully planned and scripted way through secure channels that cannot be traced to anyone related to the action. Don't just go blabbing and bragging about it. Some people will argue that issuing press releases and communiqués after actions is just needless grandstanding and is likely to dramatically increase the chances of someone getting busted for it, if only because those in authority must save face. However, if getting word out in a very careful and predetermined way can help reduce the likelihood of spurting something out during some drunken binge years later because you kept it bottled up so long, then careful and deliberate communication is certainly preferable. Most people busted for doing clandestine work get found out through their own careless chatter.
And if someone you know starts telling you more than you think it is necessary to hear, be sure to stop them before they go further. This helps to develop what is commonly called "security culture." We all love to hear stories about how someone got in a good jab at The Man, but knowing the precise details—particularly the names of the ones doing the jabbing—does not help bring Him down, it just helps Him figure out where that punch just came from in order to prevent it from happening again. The more we call each other on flapping gums, the less likely such loose talk will need to be actively interrupted in the future.
Of course, security culture has its downsides. The reliance on adages culled from military jargon should give us pause. Security culture is, or at least should be, one set of practices among many. Allowing it to dictate all of our relations with the people around us can only lead us down the paranoid path to isolation and authoritarianism. Clandestine groups and organizations (the ones that manage to remain clandestine) are notoriously authoritarian, even the most vociferously anti-authoritarian ones. Secrecy is poison to accountability, and even the most compassionate and respectful people become authoritarian in the absence of accountability from their peers.
So, engaging in a security culture, as well as the clandestine activity which necessitates it, should always be carefully measured against its pitfalls. Act upon your desires for a better world, with a healthy disregard for—and a watchful eye upon—authority, but don't let those actions be channeled by a repressive atmosphere into a lifestyle that will necessarily isolate you from the people around you who make your life worth living. We don't need any more martyrs.