I have an RT tuition tape - from Pooley's I think - which I used to play in the car - reasonable useful, but a bit stilted. The case got covered in coffee, but I think the tape still plays, from about 4 years ago - PM me and I'll stick it in the post.
IMHO, a key part of successful and succint RT is to a) relax and b) know that the person you're talking to *is* on your side...and c) when you press the transmit button, you're not talking to the entire nation - it's not Radio 4 - which is part of the solution to (a).
Unless, of course, you're in a Yak and you *think* you've pressed the right button for a private conversation with the other cockpit, make some unguarded comments, and then realise you have the wrong button, and broadcast to a much wider audience.
...especially if you use a very bad word indeed....