I've oft experienced (and heard stories about) conversation participants switching to English when noting some lack of fluidity in the target language. In a similar vein, I've heard anecdotally of speakers switching to English upon even noting an accent. I believe this is in part to attempt to make the conversation smoother or to assuage the discomfort of fumbling in the target language, but it is undesirable when learning a language.
Clearly one mechanism for persisting in the language is to reiterate the desire to practice, but I've found that occasionally it'll still drift back to English.
What else have you found to be helpful to overcome this?