Disconnect the cable clamps from the battery (negative/black first and then positive/red). Reconnect the cables to the battery securely (positive/red first and then negative/black). Any looseness in the cables can prevent your car from starting so tighten them securely.
After loosening the cable clamps and battery holder, remove the battery from the vehicle.
Begin by cleaning the entire top of the battery of dirt and oxidation using baking soda and water.
While the battery is removed from the vehicle, clean the cable clamps until shiny with the 535 or 536 brass brush. Recommended speed: 15.000 rpm.
Re-install the battery in the vehicle. Lightly coat the connections with grease and re-attach the clamps.