What are you experience with screencasts?

What are you experience when making screencasts and how did you get started? I'm considering to try it out, but I'm not sure how to tackle it. I feel sure I have some content I can deliver to an audience that would like it. Probably I have some incorrect pronunciations too, but I know from screencasts I have seen, it doesn't matter too much and even native english speakers have different accents. But I feel my main obstacle is my voice flow, I will probably think too much, "lag" while speaking or be nervous I'm not speaking enough and make it sound odd and unnatural. I know people like screencasts more than written guides.

Do you have any advices for techniques to overcome this? My plan is to make about 4 minute videos. Do you make a script and practice it 10+ times before making it, do you just talk from the head and there must be some techniques to get started?

I have already looked at microphones like Røde or Blue Yeti, but that doesn't matter so much if my flow of speaking sucks :-)