Thanks everyone.
I've got a three octave iRig Keys, and an 88 key digital piano. Between those, I'll record individual melody lines, play about with ideas in real time, and record midi CC values. I then quantize/adjust MIDI timings in the piano roll view. For some of the simpler parts, e.g. where there's one note per bar, I'll just use the mouse to enter notes in the piano roll view directly. I do all of this in Sonar.
I create a new midi track for each midi part, except for the drums where I'll use a drum map (resulting in 60 midi tracks in this project; 125 midi/audio tracks in total). I tend to group the midi parts in track folders, e.g. solo strings, ensemble strings, brass, etc. Each audio part goes to a separate audio track for maximum control when mixing; I've got all the audio tracks in one track folder. For parts that are duplicated, e.g. multiple violin players each playing the same part, I use copy/paste to create linked midi clips. I'll then put the quantize midi plugin on most midi tracks, applying a small amount of note start time randomization so everything doesn't happen at exactly the same time.
There's some parallel compression on the drums that use aux tracks to squash the drums, finally going to a bus. Most instrument groups go to a bus, e.g. vocals/brass. Some go through multiple busses, e.g. solo strings -> strings. Each track/bus will have the console emulator on it; most will use the QuadCurve EQ and other ProChannel effects, e.g. a compressor. The master uses the CA-2A and Cakewalk's Concrete Limiter gently.
Vocals and instruments have sends to different reverbs, and the vocal special effects are sends to busses.