I was repairing an electric wood chipper (from Harbor Freight) recently, and I needed to put in a small bolt with a washer inside the chopping chamber. The bolt went in vertically, and the tightness of access to the chipping chamber made it impossible to put the washer on the bolt and then screw the nut on the bolt--with one hand that barely fit inside the chamber.
I put a magnet on the back of the bolt (outside the chamber) and then slid on the washer, which held from the magnet. I was then able to leisurely screw on the nut without worrying about the washer coming off.
Tightened up, painted the threads and nut with a little fingernail polish to prevent the nut from loosening from the vibration of the chipper, and we're back in business.
To reach inside the chipper with the nail polish brush, I used a 12-inch stainless steel locking clamp (also from Harbor Freight). This type of clamp is used by surgeons, but it's also a really handy tool for the home DIY person. I use it to clean hair and junk from sink and shower drains, from garbage disposals (ever get a piece of plastic or aluminum in your disposer?), and to get into hard-to-reach places. I once used one of these to grab a bolt off an automobile engine where it had fallen. I couldn't reach it with my hands and didn't have my extension magnet handy.