Others have written about how to do screencasts most effectively. Like I said in a previous post, much of it is very useful. I just thought I'd add my own rules that I use when creating screencasts just in case anyone is interested.
1. If it's worth screencasting it's worth sounding interested in it. Try to be enthusiastic without speaking too fast. (I may be guilty from time to time here)
2. Make sure you record the screen at sensible size. I use a window size of 800x600 and I get it to this size by using a great little app called sizer (see below).
3. Always record in Flash for movies of the highest quality and easy distribution over the web.
4. Use a good microphone - I use a Logitech USB mic and the sound is crisp and crackle-free. (My earlier efforts were not so good!)
5. Go easy on the 'special effects'! Don't use the George Mallory approach. Works great for mountains but not for screen capture software.
6. Plan but don't script. Know what you're going to do but not necessarily exactly what you are going to say. (And don't worry about little mistakes - they keep it human - if you are one)
7. Work towards a standard format that your viewers recognise and trust. I find this helps comprehension and makes learning easier.
That's all for now. There's loads of other things I could have included but they are the ones that I think are most important.