I find it very difficult and uninspiring to try and "plan" out a song. It works much better for me when I let the song "come out" on it's own.
Usually I'll have either a melody, a riff, or a chord progression idea that I like and I'll just improvise around until I come up with something else I like.

Any of you interested in the creative process might want to check out "On Writing" by Stephen King..great book.