When it won't start, I'd measure the voltage on the battery posts (not the cable clamps, yet), both before and during cranking. Before, voltage should be up around 12.6V or higher. If it's not, you may have a bad alternator but put the battery on a charger to see if it takes and holds a charge for a few hours after the charger is disconnected.
A bad alternator could fail to charge the battery fully (if at all) or leaky diodes could cause it to drain the battery when the engine isn't running.
If the battery voltage is good, measure the battery post voltage while you attempt to crank the engine. If it stays high, now measure voltage on the battery clamps while trying to crank, to see if you have a voltage drop across that connection. If the voltage drops terribly while cranking or attempting to, the battery may be bad
If you've gotten this far and everything seems good, how bad are the battery cables? You can check what the voltage is at the starter when you short the solenoid contacts to try to crank it.