Mate i had/have the same issue.
Kneepads do help especially if you jot down the important bits, i have a wee lamianted cheat sheet for the ATIS and note section in which to jot enroute freq and instructions (it has quick fuel calcs and stuff on it too) Remember you don't have to parrot back EXACTLY what is said just the instructions as a quick and in-accurate example:
ATC: Yankee Hotel Foxtrot Cleared to 3500ft or below track A via B report clear of NZNS air space, once clear contact Wellington on XXX.XX
YOU: Cleared 3500 ft or below track a via b report clear Wellington on XXX daysemal XX Yankee Hotel Foxtrot.
Some tips that helped me. Speak Slowly, i tend to rattle off requests like a machine gun, this in turn makes the controller speak faster too. try it, it really works if you slow down they will too.
Have some practice circuit bashing concetrating on your calls, we had one student who had real FRTO issues and the instructors called the tower. and asked them to give him all sorts of weird instructions.
In failing that sit in a room with an instructor who is the "tower" and do some postion reports, clearances etc. i did this and it did help, yes you do feel silly but they will do it!
Always remember you are allowed to "Say Again" and if you really want to amuse "Say Again Twice"
Yankee, Yankee, Hotel, Hotel,..
