Before drywall and paint, use a flag of White PVC electrical tape and a sharpie a to identify eacvh lead. Write down an abbreviated description, like "PWR IN", "PWR OUT", KIT CANS (kitchen cans), etc.
At locations where only a small number of cables need identifying, I often just use colored tape with no writing. My "system" is generally as follows:
Red = Power in
Unmarked = Power out
Black = Switch loop
Green/yellow = To light
Two separate bands can be used to further identify extra cables, e.g.
2 x black = Second switch loop
2 x green/yellow = Second light
