There is no doubt making writing your first priority is the only way writing can become more than a hobby. I read a great blog post the other day from Miss Snark's First Victim: a blog for aspiring authors. The post link here has some great tips about treating writing as a career. It points out how you have to treat writing as a career from the start otherwise you run the risk of it never being anything more than a hobby.
I have to agree with everything the post says. If I wasn't pushing myself and forcing deadlines on myself I would still be treating it very much as a hobby and be aspiring to be a writer in the future. Instead I class myself as a writer now and know I have have to work really hard to get my goals done.
On that matter I really don't have very long until I need to send my 15 minute plays off for Famous for Fifteen so I really must press on! Fingers crossed I get them to a point where I am happy and send them off in time!