A student emailed me this question today, and I thought it was beautifully simple, direct, and honest. Here is the answer I gave her (unedited):

The basic ingredients to getting better at anything are all the same: deliberate practice. The ‘deliberate’ part of that is key, though! Simple repetition of something is not enough to improve, or at least to improve past a certain plateau. You have to focus in on areas that you know need work, and then strengthen that muscle. You also need to get good feedback (which is where I, and other teachers, come in, of course), but good feedback can also come from your own practice. There is absolutely a way to be able to tune in to the things that are and aren’t working in your own songs, and the best way I know is to simply write A LOT OF SONGS. Get less attached to any one particular song, and more interested in the process of writing.

Here are my other hot tips for improving your songwriting:

• Learn a lot of covers! Learn classic songs, but also anything that you just love. Seek to understand the chord progressions, the song forms, and the relationship of the melody to the chords and song forms. The Beatles played covers for a year in Hamburg for 8 hours a day before anyone had ever heard of the Beatles. Bruno Mars played covers in cafes in LA for 5 years before anyone knew who he was. 
• Keep getting better on your instrument. The more you can do on your instrument, the broader your palette of choices will be.
• Integrate music theory concepts immediately into songwriting (do not wait; do not think, ‘I will store this away, and use this concept next month/year…do it in the same few days that you learn something). For example, when you learn a particular inversion, or chord pattern, or bit of non-diatonic theory…write a song that uses it. 
• And most importantly, WRITE A LOT OF SONGS. Did I say that already? WRITE A LOT OF SONGS. Aim for 1 a week. You can also set yourself a timer, and make it ‘one song in one hour’ so that you are not laboring over it all week to the exclusion of all else. For 2 weeks of every year, you should have a period of ‘write a song a day’. You could pick a month each year, and do it at the same time; that way you can claim that it is your job. Draw a boundary around it. Someone asks you to do something else, and you simply say to them (and yourself), “I’m currently doing a Song-A-Day Retreat, can we do that in 2 weeks?”

Hope that helps 🙂