okay, im guessing when your writing something you kind of have a tune in your head. (if not, that could be your problem :p )

what i do is i get an idea. and then as things start rattling around and gradually start crankin, i get the lyric, and can sort of hear a tune with it. just write the lyrics along with the tune. after you got the lyrics down, transpose the tune in your head to your guitar :D

thats an easy way.