When I write songs, I don't use my guitar. I have my music come directly from my heart, my head, my mind, my body, my spirit...

I rely ENTIRELY on ear training. I do use my knowledge of modes and arpeggios and music theory to an extent, because I know which note sounds like what when I see it, but I compose without my guitar.

I use Guitar Pro! :D pressing buttons, and using my ear to hear the next note.

I've come up with very interesting chord inversions, suspensions, techniques, and all that... The coolest thing about it is that it sounds more like music, rather than licks strung together...

But I think very much about the element of Rhythm. I think that is the most important element... I've written an article about it, you might think to check it out.. http://guitarlessonsinsandiego.com/IntroductionRhythmTheory.html

It got good reviews on ultimate guitar, so you might like it :)

Anyway, train your ear, and songwriting will come totally naturally to you.. you'll be able to hear entire compositions in your head!!