A good conversation is all about the back-and-forth; both parties listening and responding.
Here are a few tips for dealing with a Chatty Kathy or Mr. Chatterbox.
Notice how I said helping, however.

So give a shot to keep that in mind as you attempt to get the conversation back on track.
It re-enters you into the conversation and adjusts their train of thought just enough to stop their rambling.
Lastly, if you know them well enough,tell them they ramble.
Remember, they may not actually know that they do it and may welcome some help.
Be polite and explain that they have a tendency to say more than they need to.
Express that you totally get them and that there’s no need for them to over-explain things.
Can I tell you what I heard you say?"
…When you interrupt, be ready to say something about what you hear them saying.
Don’t go for a deep psychological explanation.
Something simple and to the point, but if possible, something that reflects something positive about them.
Remove Their Audience
Some ramblers enjoy the attention that comes from people listening to them.
When all else fails, you should probably take away the attention they’re getting.
Your time is valuable and not everyone is going to respect it.
Sometimes you just have to say “sorry, I need to go.”
People get excited about things and some people don’t have someone to share things with.
Maybe you’re the only one they know that will actually listen to them.
You don’t have to be a pushover, but they’re only human.