I teach business and general English to adults. I find one of my biggest challenges is teaching upper level conversation students who have a good level of fluency, but with some deeply ingrained mistakes. These might be specific words that they learned incorrectly, usually confounding them with their native language (usually French), or, more often, they are incorrectly learned idioms (eg: "I think so that..."). Because they are fairly advanced, they dislike repetitive drills, but my constant corrections don't seem to be helping.

Do you have any suggestions for breaking bad speech habits?