Best way to untie that tongue is to practice your radio work while chair-flying. Or in the sim... if you're using ATC in the sim, you don't need a voice program or anything to do this... when you choose your response, say it aloud. Maybe use text only and turn off the robot voices...

I used to also listen on my scanner to RL chatter and assume the role of a flight heard on the frequency, trying to beat them to the punch on their readbacks. It helps.
In general, though, the most important thing in flight is to run the words thru your head before you key the mic... say it aloud, if need be.
And relax... you'd be amazed at the brain-farts i hear on the air, even from airline pilots...
::)
And of course, don't forget (on initial callup):
1) who you are
2) position and altitude
3)your intentions
I'm repeating myself and others with that, but it cannot be said too many times... very, very important!