An excerpt from a piece on 'received pronunciation' and the associated accent once associated with the BBC:

Performance coach Darren Smallridge, and head of professional development Bruce Wooding, gave me a crash course in learning the basics of this cut-glass accent, although their techniques were somewhat unorthodox.

For example, wedging a cork in my mouth and attempting to read lines from Julius Caesar was invaluable, helping me keep the tongue flat and speaking with restricted lip movement, but I did feel like a snake who had tried to open a wine bottle with his fangs, only to get stuck.

... of course folks are familiar with Plutarch's life of Demosthenes:

Demetrius, the Phalerian, tells us that he was informed by Demosthenes himself, now grown old, that the ways he made use of to remedy his natural bodily infirmities and defects were such as these; his inarticulate and stammering pronunciation he overcame and rendered more distinct by speaking with pebbles in his mouth; his voice he disciplined by declaiming and reciting speeches or verses when he was out of breath, while running or going up steep places; and that in his house he had a large looking-glass, before which he would stand and go through his exercises.