Very good advice in this thread.
To add to the more tangible advice, I would suggest reading up on the basics of chords and such (I'm of course not aware how much you've done so already). First and foremost, songwriting is a craft, and just like in any other art, you won't be able to transmit your musical intentions convincingly if you're not fluent in the craft itself.
I remember one thing helping me tremendously was learning about function chords.
rumborak