First inspect and clean the battery cables. Remove the cables from the battery and sand the contact surfaces of the connectors to a shine. Clean and lightly coat with grease. Then move on to a thorough wiring inspection. Look for oxidation, chafed insulation, bad crimps, loose connectors, signs of wear, etc. If the solenoid can be serviced, try to clean the contacts. If you have some multimeter experience, check voltages at all of the points along the way to the starter and for low reistance grounds.