Shit, I've kept plenty of secrets. As much as I have a big mouth on here and "other places", it seems people like to confide in me.
It's not that difficult really, just don't tell. I know many MANY guys who won't keep a damn thing from their SOs or Wives (pussies) but we all know not to tell them anything......
But it all depends on what that secret is, for eg: I don't care HOW good a friend you are, lifelong or otherwise, if I find out you're cheating on your GF or Wife? I'll be on the phone 2.2 minutes after you leave.....