I start out most of my composing by drawing diagrams. Different symbols can mean various things, but mostly I will diagram things like pitch range, texture, intensity, and gesture. I will also indicate important timings in the form as well as important structure in pitch or rhythm. None of these choices need to stay etched in stone, but I find I end up with what I envisioned more often by starting this way.
It doesn't need to be complex. Your diagram can be as simple as indicating:
Exposition: :00 to 1:00 C major to G major
Development: 1:00 to 1:30 G major to E minor, 1:30 to 2:00 E minor to A minor (big climax here!), 2:00 to 2:30 Get back to C major
Recapitulation: 2:30 to 3:00 C major
It helps me to have this plan, especially a graphic one. Otherwise composing is just noodling and rambling until I run out of steam. I'll scan some of my diagrams and post them later.