For improving writing skills, the two most effective methods I know are:

1. Participate in National Novel Writing Month. The idea is to write a 50,000 word work of fiction in 30 days, without fretting over quality too much. If you do this, you will learn a lot. It's unavoidable. (Plus, you get a book with your name on it. That's a great feeling.)

2. Write blog entries, and post them somewhere people will see and discuss them. Less Wrong, for instance, or Hacker News. If you know you're writing for an audience, it forces you to clarify your thoughts, and put more effort into writing well. This is good practice, and you get feedback; commenters on the internet are usually pretty blunt.

Each of these techniques exercises a different type of writing, so they're complementary.