There are a host of good training options out there. Sharebird is a good place to start and you can google alternatives as well. I mentioned a few books earlier on as well - Al Ries and Jack Trout and it's called "Positioning: The Battle for your Mind" and "Obviously Awesome: How to Nail Product Positioning so Customers Get It, Buy It, Love It" by April Dunford.
Other than that it really comes down to experience. Just do it and get better and better at it as time goes on.