NFPA "Electrical Standard for Industrial Machinery", ANNEX D, illustrates one common method. Each conductor is identified by a unique number, assigned in sequential order, or by using the drawing's line reference. When using the line reference system, each conductor on a particular line is identified, XXXXA, XXXXB, XXXXC, etc.
Standard in the electrical power category is 5 or 6 digits. For example:
Wire # PPPLLW
PPP= Page of the drawing
LL = Line of the drawing
W = wire #, counted from left to right
As for me, I make up my own method depending on the complexity of the job. If it has less than 50 wires, I start with Wire #1 and add 1 until finished.