Want to be a more interesting writer? Then lead a more interesting life.
It's that simple - and that difficult.
Writing can be a very solitary occupation, and most writers grow to be content with being alone for long stretches of time. While I think that this obsession with writing is pretty much a requirement of the working writer, it can make you dull. And that's a problem.
You might not even notice it at first since your previous life experiences have given you plenty to write about, at least for a while. But sooner or later you need to get out in the world and refill the creative tanks.
So remember to take a break from the keyboard and go do something. Anything. Interact with people, play basketball, go to the pub, it doesn't matter. Interesting, involved people have interesting stories to tell.
So be interesting, and your writing will never be dull.