Character Voice Development: Maintaining Distinct Voices

I’m working on a fiction audiobook with 12 different characters, and I’m finding it challenging to:

1. Create voices that are distinct enough for listeners to recognize

1. Maintain consistency throughout the entire book
   
2. Keep track of all the voice characteristics
   
3. Not strain my voice with unnatural techniques

For those of you who narrate fiction with multiple characters, what’s your process for developing and maintaining consistent character voices? Do you use any special tools or systems to track the different voices?