The Economist Style Guide'

http://www.mantex.co.uk/2009/06/16/the-economist-style-guide/

The Economist is rightly renowned for the
clarity of its house style. They have an
online service offering advice on good
writing practice.

It includes tips on how to avoid common
errors and solecisms, and an online quiz to
test your writing skills.

http://www.economist.co.uk/research/StyleGuide/

If you want the complete style guide in book
form, have look at the details here -

http://www.mantex.co.uk/2009/06/16/the-economist-style-guide/