Assuming battery is good, check that battery terminals are clean and tight. Follow earth lead to where it bolts to the engine and check its clean and tight, also that lead is not damaged. Crimped on ends can corrode causing bad contact. Do the same with the leads to starter solenoid and to starter motor. Check that the solenoid contacts, the internal contacts tend to develope a scale due to arcing when making and breaking.
Mr. Toad
August 2008