Effective writing skills are to a writer what petrol is to a car. Like the petrol and car relationship, without solid skills writers cannot move ahead. These skills don’t come overnight, and they require patience and determination. You have to work smart and hard to acquire them. Only with experience, you can enter the realm of effective, always-in-demand writers.50 Free Resources That Will Improve Your Writing Skills | Developer’s Toolbox | Smashing Magazine

Most useful to those who write in English but some of these resources contain good general advice to any writer. Jolly Good!