Used to run cable above drop ceilings with a glow rod, now I use a 3 foot piece of chain and a magnet from Harbor Freight | Cable Installers - Guest Forum
Wrapped a small rare earth magnet in tape and tied it to mason line for the same reason. The chain trick works great until the magnet snaps onto a screw or a metal stud and you have to fight it loose. Pro tip if you go that route, tape the magnet so it can't slide off the chain when it grabs something hard. Also watch out for ceiling grids that have wire ties up there, the magnet loves to grab those too.