One element that seems to be missing from the posts so far is that songwriting is like anything else...hard work. Like Bob said above, sometimes it just happens and for those times thank your lucky gods! Most times, at least for me, the first time is too cliche'. In a typical song I may have a nugget or two that is worth keeping but most of the rest is garbage. After a few rewrites it begins to come together.

A lot depends on the genre as well. Trying to keep it relevant to today's audiences can be a real challenge. I seem to always migrate back to the 50's and 60's, be it blues, rock or country. If you're writing for yourself then that's ok, but if you are working to sell the song, that just don't cut it.

Takes a lot of hard work, and lots and lots of patience and practice, just like anything in life that's worth anything.