QUOTE(mcm @ Jun 15 2007, 07:58 PM)

Have you tried wrapping a cloth dipped in hot water around the mouthpiece receiver? It might expand it sufficiently to release the mouthpiece. If your trumpet is lacquered though, too much heat might damage the finish.
Well, mcm beat me to it, cos that was the sort of advice I was going to give, having experienced similar problems with all sorts of equipment.
What works best is to use the principle of cold=contract, hot=expand. This has always worked for me. Even with stuck clarinet joints!
Soak some tissues in ice cold water and stick them down the MP and around it. Leave 5 mins and repeat with fresh ice-cold tissues. Boil kettle.
After 5 mins, hold the 'tight' bit of the tube over the steam for 30 seconds, then twist the MP out.
If you're worried about the laquer, use hot tissues around the tube. Should work!
Steve