This will be short and sweet,so here goes.How many times 
have you been writing a song and while your building
lyrically you have some what of a melody in your head but
when it comes time to add music you can't quite seem to find
the notes or chords?I came across a little time tested trick
that has helped.Begin with a cheap tape recorder and as you 
read the words silently, 
hum the melody(not the words) and record it .
make sure to do it in a key you or whomever you have in mind as
your vocalist would be comfortable singing . 
Next go back with instrument of choice 
(DUH! Guitar) and play along until you find the notes.
I know this may sound silly....
but I find it very helpful.
It will give you a actual audio
guide to copy....rather than trying to 
magically transpose what's in your head.
Hope this may help somebody.
TODD