If there was a proven formula, we'd all have more traffic than we could handle and we'd be sipping drinks on our own private island. Plus it can vary per market, product, service, audience. Depends.
I'd say...
First is the headline. It has to draw them in. Doesn't matter how good your content is, if no one reads it, right? So many theories about headlines and there are some guide lines, but usually it comes down to testing and tracking. Course once you have some data, over a period of time you could probably come up with better performing headlines in the future.
Then of course the content itself. Again, it varies per market or audience. Guess it comes down to really knowing your target audience and what they want. Then it shouldn't be as hard to come up with content that resonates with them.
The content could be very useful, entertaining, controversial, news worthy or funny.Again, takes knowing your audience inside and out. Something most marketers don't do enough of, including myself.
Just my opinions. Hope it helps.