Writers should be paid for the spaces they leave - not by the number of words they use. Think sound effects and think music. Don't write over action. Set it up, then shut up. Always ask - why is that sentence there? If it isn't a signpost or an interesting piece of information lose it. Let people watch the pretty pictures in peace.

Some people will tell you that the words in the commentary tell the story. Perhaps they should write for radio. Television is a visual medium. Let the pictures tell the story. How many times have you heard "The antelope looks up in surprise"?

If you can't read it - how the hell is your narrator going to manage? It also forces you to use simple words. A script needs potent words, not big words. A script needs sexy phrases, not scientific cleverness.

Structure sentences so that the most important phrase is at the end, both for facts and for links to new scenes. The listener is more likely to remember the last thing you say. For example: "In his lifetime, a man will produce an incredible two thousand billion sperm cells!" is more powerful than: "Two thousand billion sperm cells are produced by a man in his lifetime."

A factoid is a fact presented in its most interesting form. Convert dull figures into analogies that your audience can relate to eg. How would you like to be able to glide 100 times your body length? That's the length of not one, but two jumbo jets!

Don't try to tell too many stories. Identify what the pictures allow you to say. Even the coolest stories get lost if it's wall-to-wall voice over.

Sentences need to be short and simple. Narrators tend to hyperventilate on long sentences. Listeners don't get a second chance to re-read a sentence to get the meaning of a subclause.

Just like telling a joke - things sound better in groups of three. For example: "the good, the bad, and the ugly".

Could it be that rhetorical questions are the most potent device in the writer's toolbox? Isn't it better to tease a thought out of a viewer than blandly state a fact?

Spoken English is different to written English. Use things that sound nice: alliteration (repetition of consonants); assonance (repetition of vowels); abbreviated expressions (two word sentences); use colloquialisms; use abbreviations.