Berkery trained Renee Zellweger to sound British for her role in the movie “Bridget Jones’s Diary.” In this article on the NY times, Berkery says before filming started, Zellweger used an alias to get a temp job at a London publishing house where everyone assumed she was British. Order at a restaurant or ask to try on clothes in a department store in your new patois. “Use the accent with strangers, otherwise you’ll never get it,” Berkery says.
This is great advice for learners of any accent in fact, trying out what we've learned in a real life situation. If we don't use it, we will never know how we sound.