Practice writing songs daily. Every day you should try to come up with a new song idea or continue developing a song you're already working on. This will keep your creativity sharp.
Take professional lessons to become better at playing your instrument. If you are limited as a player, you will be more limited as a songwriter. Being a better musician will give you more possibilities to work with when you're writing.
Study melody and chord progression. Melodies and chords are the foundation of any song. To get a better understanding of how they work in relation to each other, enroll in a music theory course at your local community college or check out books on the subject from your library.
Train your ear to clearly hear and recognize pitches and chords. You can do this by learning how to internalize music. There are many ear-training courses available on DVD and CD.
Learn how to play the music of others. Study your favorite songwriters by learning how to play their songs, and think about the process the songwriter might have used during the creative process. This is the best way to learn new techniques you might not otherwise discover on your own.
Write your song lyrics and chord progressions in a journal. Review your journal daily or weekly to track your progress and look for improvements in your writing. Evaluate your strengths and weaknesses as you revisit past songs you've written.
Record your songs so you can hear what they and you actually sound like.