Immersion is a good way to force a person to write more. Writing more will almost surely lead to a big improvement, especially if a person has never done that before. The next big Internet event is in February, write an album in a month. It is a community. Most participants are guitar playing singer-songwriters. However, there are more than a few instrumentalists and other kinds of songwriters and composers. It is also a good place to find collaborators and to get exposed to a lot of creative people.http://fawm.org/
The subconscious mind is powerful. If a person joins a group where everyone sets a goal of writing and recording 12 to 18 works in one month, the subconscious mind can take a person there. Yes, I know it seems impossible for most, or unpleasant to think about churning out that kind of output. And yes, there will almost always be some low quality work in the mix. The other side of this is the ratio of "good work" usually remains near constant. So more output means more good work.
I will again point to Bach, who had a job where he was expected to compose, transcribe with primitive equipment, and have rehearsed with the musicians, 25 minutes of new music every week.
A similar immersion group is what jump started me as a songwriter. My participation in 50 songs in 90 days, took me from writing one song a year, only when inspired, to writing 25 songs in 2 months. Like others reading this, I thought it impossible before I tried it. Really I had no intention of writing so much, I just was curious what others were writing. Once I joined, the creative juices started flowing. Setting aside blocks of time every day were another key.
Other things a person can do are to take music appreciation, music history courses. There are also music theory, and composition courses and books as well. Some are online, some are at local schools. Each person will find varying value in the books and courses. Personally, I favor music appreciation and history. The theory usually gets too heavy into jargon (which makes my eyes glaze over), and composition too heavy into sheet music (where I am at a first grade level or below so can not follow the discussion).
The analogy I use is that studying will give a person more vocabulary, but the act of writing will improve a person's writing skills. There is a balance in there, especially for novices. However, most would-be songwriters and composers would do well to write more and study less.
/edit to add:
For those interested in courses, here are two that I have done. The first is a free course focused on classical music appreciation:http://oyc.yale.edu/music/musi-112
The second is a multipart (like 20 parts) 10 minute per segment, series on Youtube. I believe it was broadcast on PBS and/or BBC. The link is to Part 1 of How Rhythm Workshttp://www.youtube.com/watch?v=c_jEkNiYFNc