Cogy,
Grummaniser and Mister Geezer give very "sound" advice (excuse the pun!). I was a marine radio officer for 10 years but the radio patter in aviation is totally different...for a start it's spoken, not hammered out on a morse key! I had a lot to learn.
I had several years of airband monitoring with scanners before I took up flying. When it came time for me to go "live on-air" (my first solo, actually) wonder of wonders, all the standard phraseology and delivery was already formed in my mind. Engage brain, open mouth, and it came out just fine.
My instructor told me the only fault he could find with it was I was too quick...a habit formed by years of listening to very proficient heavy-jet pilots.
Good diction, steady speed of delivery and being absolutely sure of what you intend to say will see your radio skills right up there with the best. As suggested, montoring the field you fly at, or your local flying-club field is probably gonna be more help than listening to the heavies. In any event, beg, borrow or otherwise obtain a scanner and the tuition is free!
The tower visit is a very good idea. A lot of controllers are keen to meet pilots because it helps bridge the gap and you get to appreciate their perspective on affairs.