Kite String, velcro, zip ties.
I like the pipe coverings idea and the CC foam for lifting cables from the floor. Now that one I'll try despite the fact the wires will bitch about laying on a lumpy surface rather than a smooth one.
I've seen velcro strips online with adjustable rings thru which one can trace cables up.
Usually though, I resort to string or zip ties on my six shelf rack. On the cheaper Wally world rack I just use tape and self adhesive conduit I cut to fit. That one holds all the cheap stuff though escept for a couple items.
True too... there is only so much you can do when a big rack is filled up. you can't put just anything anywhere on mine either. I ran out of room real fast.
Addi in the fact not all the power inlets on components are on the same side and likewise the signal connections, and it can be a mess back there. Especially if you run SE vs. XLR.
More importantly I'm now run out of dedicated ckt/outlets. Amazing.
Pipe insulation. Hmmm. Good.