I've been using it to develop live presentations with slides, and was very helpful in working out a talk for Ignite Phoenix, where timing is extra crucial.
I tend to write down some notes, then do a recording more or less ad lib, see how much I screwed up and flubbed things, and gradual get a feel for what phrasing feels most natural, what steps are needed, what I have to explain.
The end result is a sort of pre-recording of your talk, which you can watch as an aid to practising and getting the whole thing into our head.
Also, good point on keeping screencasts short. I hate having to jump around in some 20-minute recording trying to find where one or another item was explained. Better to break things down to a set of tighter presentations.
I tend to do everything in one take then edit out the mistakes so I haven't got an informed preference, though doing the audio first does seem to make sense from a pacing and "maintaining interest" perspective.. you can always edit down/speed up the video parts, after all.
I suspect that merely being detailed enough to think about these processes and how they work (or not) for you is enough to put you above average in the screencasting stakes.
