Usually I start with the music. I'm basically a rhythm player so 9 times out of 10 I start with chord progressions or a riff and a beat. If I get a really good groove I try and come up with a title and start writing from there. I have long list of titles I've collected. Most of the time a chorus suggests itself. I try not to be too formulaic but if I loose interest while listening to the song I'll try and figure out a bridge to break up the monotony. I also work with different instruments (midi keyboards) so I can arrange out much of the monotony by introducing new sounds where appropriate.
I try to remain flexible during the process and much of the time I end up far from where I started. Occasionally I like the end result.
