Color-Coding Vinyl Electrical Tape 6 Pack (Professional Grade).
If you have a large number of cables, this tape is excellent for keeping track of them. Wrap a little tape of the same color around each end of a single cable, and use a different color for each individual cable - then you'll be able to identify the set on sight! You can also create little tags to write on if you need extra identifying information about a line. This may come in handy if other techs need to work with your installation, or if you revisit the installation much further in the future.