How To Make a Living Writing

Also, don't forget to "Develop a tough skin." As any writer knows, you have to be able to accept criticism of your work. Oftentimes, it's constructive. But sometimes not. Regardless, criticism will help you in the long run. We're all blind to our blindness, and we need a good editor to help point out the things we can't see. Over time, you will develop a callous and will see criticism and feedback as something that is helping you become a better writer - not as something that's meant to attack your person (in other words, learn to get over yourself and don't be so in love with your own writing). Because improving your work and sharpening your skills is the end goal anyway, right?

And then you have to "Understand the medium." Writing for magazines is different than writing books is different than copywriting for an ad agency is different than writing for the Web, etc. Study the differences and figure out where your writing (and your "voice") is best suited.

I fully agree. But feel that you should add one thing.
Having written 50 odd plays for Christian Ministries, I must also add that the most important invention of the 20th century is the spell checker. English is a dreadful language to spell in, having spelling rules from more than a dozen languages to flounder about in. Do use the spell checker on your writing. I am constantly amazed at the horrible spelling in e-mails and other documents I am sent that give the general impression that they were written by illiterates.