i personally use mostly MIDI for songwriting.. its a good way to get yer ideas in the computer without having to record them and you can alter them any tim e you want - whenever i me up with a melody or a lick or a rythm part on the guitar i immediately programm it into the computer (whenever i am close to one) so that i will not forget the idea. i name it only by "idea03_13_02" (date) and save it in a folder along with all the other ideas.

then, when i need riffs or whatever i just open that folder and look what i can find.

the next step is to create the music around it - i also do this by programming midi (with the assistance of my guitar and keyboard) and slowly i put a song together.

then i do the same thing as schmange said with the lyrics - only "blahblah"-improvisation and then i (or someone else) come up with some lyrics. and then off to studio ;)