When i try and write something original i start out with a basic approach.
Lyrics to get me started then i will hum a few chords to myself and see what fits then build from there.
I find that writing the intro music hardest..i tend to over do it sometimes but i am glad i have band mates who are on the same page as i am when it comes to writing most of the time.
One thing i can suggest is getting a little pocket recorder for when you get something in your head. you can sing it and record it which will help you later when you can sit down and work on it some more.
anyway... thats all i have for now.