How to Pull Cables with String

The knot to use is called a "stitch", and it is the same technique used in data centers to secure bunches of cables without all of that Velcro mess.
Either the "Right" or "Wrong" style will work, and I describe the "Wrong" method below. This works for Ethernet cable, Romex®, and similar cabling. It's a great technique to use with fish tape, the metallic band on a reel used to fish inside of walls.

1. Make two loops in the string.
2. Turn one around.
3. Stick the cable through the loops.
4. Pull on the string to tighten (it's OK for the loops to separate). If done properly, the harder you pull on the string, the tighter the knot gets. If done wrong ... just do it again.
   
5. Wrap with as little electrical or duct tape as possible to secure the loose end of the string, protect your cable end, and to keep the end of the cable attached to the string so it doesn't wedge as you pull it. Too much tape will snag as you pull it through.

For fish tape, you typically fish the tape from where you want to go to where the cable is. Attach the string to the hole in the end of the tape. The other end of the string gets the procedure above.

Sometimes your intended run is such that you can't get yourself or the fish tape reel to the destination. If you have an open space between two holes, sometimes the fish tape flops around. In those cases, attach the string to the fish tape before fishing it through, so that you can control the tape a bit with tension on the string. You will then have string all the way through the run.

I've used this knot (doubled) for connecting two ropes, since other knots form a point of weakness and this one doesn't (when doubled). Knot the loose ends in a square knot rather than with tape.