My best advice is to let them know you have a stutter. It can be embarrassing stuttering in front of a manager or a superior, but unfortunately at most jobs (especially corporate) you cannot get away with not talking. With that knowledge, they can either use different forms of communication (email) or be patient when you're blocking or stuttering. In my case, If I find myself blocking when talking to a coworker, I'll stand there uncomfortably until I finish my word/sentence.