I think, honestly, it's a trial and error process for everyone. Someone else has said practicing beforehand helps them, but for me that makes me block more - it's much easier for me to get words out if I improvise and 'surprise myself' with the words I say. I think it's all quite a personal thing. Something which I think applies to everyone is that you really need to be more forgiving with yourself. I completely understand it's frustrating (of course it is) but frustration will only make the blocks stronger. Trying to calm down, understanding that stuttering is not a crime and it's an acceptable way to speak - all of this can help reduce the pressure and anxiety that makes you stutter more.