Not an expert so take this with a HUGE grain of salt.

I think that the answer might depend on whether the difficulty is something with your speaking itself (e.g. you have a speech impediment, accent, stutter, etc.) versus if the difficulty is with the social side of things (e.g. mumbling due to nervousness, etc.).

For the former, maybe a speech therapist could help? For the later, I've heard people say that improv classes/clubs can do a lot for helping people be more confortable in unpredictable social situations.