I'm considered pretty good in this respect. I think the #1 thing that helps is just paying attention to things a lot and having a high degree of situational awareness, which causes you to observe more interesting things and thus have more good stories to share. Reading quickly also helps.

When it comes to actually telling the stories, the most important thing is probably to pay attention to people's faces and see what sorts of reactions they're having. If people seem bored, pick up the pace (or simply withdraw). If they seem overexcited, calm it down.

One good environment to practice the skill of telling stories is tabletop role-playing games, especially as the DM/storyteller/whatever. In general, I think standards in this field are usually fairly low and you get a good amount of time to practice telling (very unusual) stories in any given session.