Usually I bring it up early in a conversation if I see the other person doesn't understand what's going on, or whatever. Sometimes I'll work it in as a joke, or sometimes I'll be more serious about it. Depends on the person I'm talking to obviously. I'd say being upfront is probably the way to go, especially if you block hard or have trouble with certain words/vowels a lot, like me. Keep on keeping on, the people who listen to you even though you're having trouble speaking are the ones who really matter. If people don't have time to listen to me talk and share my ideas, then I'm much less likely to pursue interaction with them in the future. Just my two cents from a lifelong stutterer, at thirty years old.