When I say to keep your mouth shut, I don't mean to stop talking, I mean that you need to keep quiet about a few things:
Your opinions of others.
Shitty things others have done.
What your plans and goals are.
Who is doing what.
Baisically, there is no business, like minding your own business. The guys I do know that are 25 year olds gossiping like little girls, get zero respect. People don't want to associates themselves with them, help them, or hang around them longer than their stories last. So much so they usually get cut out of social gatherings.
Keep vague. If someone asks you something that pertains to these things, say it's "good." Say it's "going well." Say, "I don't know."
When you speak on others in your social circle, whether they're close with you or not, you take away from yourself. You also portray yourself as someone who gossips. People won't trust you or want to be associated with you. I know some guys who can seriously work on this who are alphas in my circle. They just don't have interesting shit to talk about every second and start going off about other people's business. Makes me sick, makes me know I can't trust them with shit, and I need to keep my eye on what I say around them in case it is communicated to others I care about in my social circle, in a negative light.
Now, sometimes, I have information that is rather exciting, and I would like to share it with others in my group, my male friends. However, I don't. I started biting my lip a long time ago when it bit me in my ass. Word travels quickly that you talk and what you say does make it back to people, and typically not in a good light. You don't want this, you're not a little girl. You're a fucking adult man amongst guys who act like children.
In the case that you do want to talk bad or at all about others, make sure it's calculated and for a reason. If you are to say something like this, it can be in a fashion that sabotages others. If you do, you need to play almost stupid and drop subtle hints about the person you are sabotaging, to whoever it is you're telling. Don't seem as though you have something to say, drop it in a conversation. This is particularly useful in your workplace and environments where you need an edge or a better image to be translated.
When people bring shit to me like this in conversation, I don't have much to say. I don't share it, and I don't feel the need to make comments on it. I typically laugh and acknowledge what they're saying.
I wish I didn't have to write this, but the world is filled with guys who act like gossiping bitches who need validation and need to make themselves feel good in order to keep status quo in their mind.
EDIT: For those of you who haven't picked up on it yet, this applies mostly to people you cannot trust. You don't have to apply this as severely or at all to your top 3/5 friends that you know hold what you say in confidence. This applies to those you know you can't trust with information and actions.