I would recommend that you get some calling DVD's, CD'S Cassettes etc and listen to those and then you can hone your skills from there. That gives you the luxury of listening to it done right and you can revisit it over and over again. I learned to call from a Primos cassette. Im not saying im the best caller in the world, but i have certainly put in my time and have become pretty profiscient with my call. I also commend you on taking the time to become a good caller as there are all too many that wont take the time to practice until opening morning.
After that, then knowing when to call is the next and a really important point.