Just remember that writing a song isn't an almost mechanical process, you can be tinkering with the same idea for weeks and weeks and still not be pleased.

Not to say the comments here haven't been anything short of excellent, but i just think great musical ideas can be produced in more free flowing ways.

Its like sometimes I can just hear an idea in my head, and it sounds so good I emulate it onto guitar, whether it be a simple progression or a short riff, mainly what I hear is a mood for the piece, what kind of atmosphere i want to create for it.