Get a scanner and listen into ATC at a nearby airport/airfield. Become immersed. Listen to others. Quickly you will be able to qualify who sounds bad, who sounds good and critique effectively. Which should improve your own RT.
That was my thought. Invest in a handheld and park by an airport with a thermos of coffee and a sandwich. The handheld is an education tax writeoff and a good piece of kit for the future. Killing two or three birds there...