1. Character cards
While this can be done electronically, old-fashioned 3x5 or 4x6 index cards work best. For each character who gets a physical description of any amount, you create a card that contains as many of the following details as you include in the text:
- Full name
- Hair color
- Eye color
- Tics and quirks
- Scars, tattoos, piercings
None of these needs to be exact. Age could be mid-40s, or even middle-aged. Hair color could be dark or more specifically reddish-brown or blue-black. It's whatever you've chosen to include so that reader can create a visual image of the character.
The point of the card is to keep track, over the long haul, of details. One of my earliest manuscript clients had an intriguing lengthy novel about football in which the main character had a robotic double. There were only three main characters but there were several dozen secondary characters in the book's various sections. I was continually finding name changes (Ann, Anna, Anne) for these secondary characters as well as changes in hair color, quirks, even the location of a scar.
The book had been written over nearly a decade in the man's spare time. He would work on it for several months and set it aside. His short-term memory had obviously let go of all the details. A stack of character cards would have helped him keep track. While I could point out the discrepancies, he as author needed to make the decision.
There's more good information about character studies and character cards in Robert Ray's Weekend Novelist.