This is always a tricky subject to master as it's all about balance.
If you (like myself) have a fabulous habit of going off on a tangent or rambling and going completely off track when left to your own devices, write down a basic list of what you need to say and work around that. It takes a few tries, believe me, but you'll be thankful you did it.
Obviously the main issue with scripting is that even if your words flow nice and evenly, you can end up sounding really mechanic. I recommend the bullet point list - know what needs to be said and then work it into your natural way of speech.
Takes some practice, but just keep at it and eventually you'll become more confident and comfortable, and find a style of recording that works for you!