Write every single day. The brain is a muscle, and like any other muscle in the body, there are healthy, efficient ways of strengthening it. The mind needs to be fed, stretched, exercised, and rested. And just like doing 1,000 crunches once a year won't get the beach body abs everyone strives for, sitting down to write a song once in a blue moon won't yield the results you're looking for.
I'm not suggesting that you need to complete a new song every day to become an accomplished songwriter, but there are many ways you can implement some simple daily exercises to strengthen the writing section of the brain. The most simple of these excersises is to sit down for at least a few miniutes every day and write something; write a song, a poem, a limeric, anything to get the jucies flowing in the writing portion of the brain.
Once you get into the habit of doing that, your next hit-song might just be around the corner.
Think back in your life. When were you the most disciplined? What happened?