Hi Rich,
I usually play brass on guitar first as a lead line and then convert it, bass I just play the song, run through ideas and then record a section when I like it. I might then mute that and try a variation on another track and see what works best.
Most of my ideas are recorded into Voice Memos on the iPhone, but as PaulyX said, I then have to work out what I did when I get back to it. Video might be the answer!
Pete