Typically, what I'll try to do is work the song into a corner, that really doesn't seem like there is any more progression to work with. Then, I kind of intuitively work my way out of it. I'm probably making it sound easier (or flakier) than I mean to, but what I do is kind of ask, "What does this song want?" If I'm frustrated, the answer never comes. If I'm patient, however, I always find a solution.

Sometimes, in this regard, my ego can get in the way, a bit. I may have a track of music that I am particularly attached to, in the over-all song, that just doesn't fit in with the rest of it. It's probably something technically brilliant (that I think of as such, at least) but just isn't working with the rest of the music. I'm often surprised at how when I get rid of that thing I thought the song couldn't do without, how easily the rest of the pieces fall into place.

Of course, there's no substitute for practice. Sometimes I'll set up a simple loop, just to work out my fingering or jam on the keyboard with. It's fun and you can experiment a lot, that way.