First off,,take apart the solenoid,, and thoroughly clean all parts
,, and use steel
wool on copper parts
until they shine, look for obvious wear on copper parts,,and where possible install part the other way around if the cut out is the same on both sides.
Then re-assemble, grease the corkscrew splineed shaft,,and re-instal most of the time this does the trick.
most starter solenoids work on the same principle,,here is a link with directions and pictures: AircooledTech.com - Starter Solenoid Repair