Almost all of my improvised music could currently be described as through-composed. The notes flow from one to the next, and sometimes I stick to a time signature (almost always 4/4). But that's the extent to which my music is structured.
Most songs I'm familiar with have some sort of song structure or musical form. I also tend to think of music as fractal (likely due to my experimentation with computer-generated music). So I'd like to at least be able to have a sense of a larger structure over my improvisation, in case it turns out to be useful. As I see it, there are at least two possible approaches:
1) Learn more about music improvisation, particularly what practices an improv group uses to coordinate themselves.
(1b: practice playing music off simpler notation (eg. fake book) more often, to a) get a more succinct view of the larger structure of common pieces of music, and b) become used to being able to visualize the larger structure, while simultaneously improvising the smaller bits)
2) Learn about the very start of the process that non-improv composers use when they write music. Then I can do the same, but use that as a launching-off point for imrpovisation.